I appreciate everyone's help and opinions here, and want to report what we ended up doing.
We (the Town of Groton Massachusetts Trails Committee) looked at a variety of options. At first the plan was just to do what everyone else was doing in the backcountry (where you can't drive a motorized vehicle to), which is overwhelmingly plain routed wood. After becoming aware of the two-colored plastic material and getting the responses here, that's what I thought we'd end up doing, and the Trails Committee was generally accepting of that.
Getting such signs made for a affordable price turned out to be the difficult part. I contacted a bunch of local sign companies. There are a surprisingly large number of them, at least here in north-central MA. Of the 10 I contacted, 4 never returned my call, and 4 more turned out to be just design services that get their signs made elsehwere. That left two places that I talked to that actually did the sign fabrication.
The problem with commercial companies like this is that they have a significant setup charge, which is a problem since each sign at each intersection is unique. Skipping over some details, it would cost around $120 per sign, plus material, for getting signs made this way. The plastic material is expensive (but worth it in longevity), but only available in 8x4 foot sheets, else the price goes up fast. That means you have to have a place to store it and deliver blanks to the sign shop yourself when you want signs made.
At $120 per sign, even just 10 intersections would cost $1,200, and that would be a tiny fraction of our trail system. We have about 115 miles of trails, and probably a few 100 intersections. All of them don't need signs, but this was still too expensive to deploy this system broadly.
Then I looked around for who might have a suitable router we could use for free or low cost. My first call was to our local high school. They were quite happy to support the Trails Committee and would have let us use the equipment when not otherwise in use. However, it was only meant to fabricate small things, had only 6 inch travel, and was enclosed to that larger things couldn't stick out and be done piecemeal:
Then I contacted our local vocational or "trades" high school. They had the appropriate equipment. I had a meeting with the guy in charge of such things, and he was quite willing to route signs for us for a nominal fee as long as he didn't have to store any material for us. It was near the end of the school year, which is a busy time for them, so he asked me to contact him again in 2 months. I did and left several messages and emails over a few months without ever getting a single reply. Eventually I gave up.
A guy I knew with his own machine shop was willing to route signs for cheap on the side as long as I could just give him the program for his numerically controlled machine. I was going to generate the graphics with my slide making program anyway, which is based on vector fonts. I figured it wouldn't be that difficult to have it output control commands for the machine, and was willing to write the software. I took pictures of the control head of his machine to track down the manufacturer:
I did finally talk to the manufacturer's technical support, but they refused to give me the required documentation. This machine was manufactured in the 1980s, and I think they just didn't have the documentation anymore.
About this time I found out that the Massashusetts prison system has a formal program where inmates manufacture things, which are then sold at very low cost. You pay for the materials, and the labor is practically free. There are 9 prisons, and each one has a particular type of shop. It turns out that one of them has a sign-making shop. This is apparently how road signs all around the state are made. The organization is called MassCor, and they even have a formal catalog. The only real limitations are that they won't sell outside of MA, and you aren't allowed to resell their stuff outside of MA either, neither of which were a problem for us.
A 12x6 inch aluminum sign with retroreflective white background and one color paint anywhere you want over that background costs about $7.50 plus a $10 silkscreen charge per unique artwork. Since each trail sign is generally unique, that comes out to $17.50 per sign. If you have them sent to a official government address, then shipping is free. Now that's a price we can afford, even for 100-200 intersections!
Signs are available in a variety of sizes, from 6x12 inches up to 4x4 feet. The latter costs $130, plus silkscreen charge. They also have triangular and circular shapes available. The foreground colors are those you find on highway signs, which are white, yellow, red, blue, green, and brown.
I want to make it clear that these are decent quality signs on 80 mil (2 mm) aluminum with paint intended for outdoor use. The 6x12 inch signs are quite stiff, and should last a lot longer outside than routed wood signs.
Here is a example of one 6x12 inch sign MassCor made for us:
We are very pleased with the quality. We initially did a test of three signs, each 6x12 inches. One had 4 lines of text, one 5 lines of text, and the third was a resolution test. I wanted to know what I could rely on their process doing, keeping in mind that some day I may want to make a large sign with a map or other graphics on it, and I wanted to know what size line/space their process could reproduce.
After deploying both the text signs at real intersections in the woods, we decided that the 5 line format was plenty readable from a reasonable distance. This is what we are going forwards with now. The sign above is from the second batch of 9 signs. We tweaked the arrows to make them a little fatter than the first batch, and added our URL to the bottom.
For the resolution test sign, I made 7 patterns with line/space widths from 25 mil to 100 mil. I chose 25 mil as the smallest because I was expecting their process to be unable to reproduce that, and it's finer than we needed anyway. To my surprise, even the 25 mil (0.64 mm) line/space details were cleanly reproduced, better than you can see in the resolution picture allowed on this web site:
While this is a good answer for us, and is what we are going forward with, it's not without issues and may not be for everyone. Getting technical specs was a tedious process. They are used to getting PDF files or Word files, but I wanted to have full control over where every bit of paint goes, and be able to squash and stretch fonts and spacings to make everything fit just right. I could have done these text signs more simply, but I wanted to pave the way for possibly doing arbitrary graphics with maps and the like. After some wrangling, I was put in direct contact with the person running the sign shop instead of having to go thru the sales people at MassCor headquarters. Eventually I got necessary specs, like the sizes and locations of the mounting holes, the radius of curvature of the corners, and the minimum required paint-free margin around the edges. Now I have templates for all that in my slide making program, and laying out new signs is fairly simple. We worked out a process where I send them a 400 DPI bitmap with white for the background and black to indicate where paint should be.
So far each order has taken much longer than originally stated. That's not a problem for us since the woods will still be there 6 months later too. Keep in mind that the people doing the work have a lot of time on their hands, like 20 to life.
I have no idea what is available outside of Massachusetts, but every state has to make road signs somehow and have felons that need to be kept busy. I'm guessing that something like MassCor is common, but I haven't looked around.
Our Conservation Commission has now formally endorsed these signs for use on their properties. We are starting there, then will approach some of the private land owners about using the same system on their lands too.