Before I answer your question about variable stuffing, I think you could do better for your basic sources on MYOG quilt design and construction.
Some good info on quilt design
The benefits of Karo Step baffles, plus design tips
Calculating down fill requirements
Some innovative ideas from Colin Ibbotson
Useful videos on construction
A comprehensive walk-through of a Karo project
Time-saving tips for sewing Karo baffles
A good source of down
For the down itself, I'd recommend DownLinens Premium Washed Down with the Water Resistant (WR) finish. This is the down used by many of the top manufacturers, directly from the source. The WR finish is the best on the market, according to many industry insiders. Not cheap, but very high quality.
A variable stuffing quilt?
I don't think many people do this, and there is a pretty good reason. Quality down is difficult stuff to handle, so adding and subtracting it from your quilt would be a pretty major faff.
Colin Ibbotson is known for his innovative gear designs, but even he couldn't think of a way to make it practical. He considered "pods" of down contained in gauze, but rejected this on weight grounds. I also can't see how pods would allow you to distribute and loft the down properly.
The main benefit of a sealable opening might be to overfill the quilt gradually as the down deteriorates. This could extend the life of the quilt over the years at the cost of a little extra weight.
For comfort, a Karo Step design allows you to move down around the quilt very easily to add or reduce insulation as the temperature changes - just work some down to the edges if you want to cool down. Plus a quilt is inherently quite flexible as you can fully or partially cover your body at will. So the only real penalty for carrying an over-specced quilt is weight.
My personal approach is to build a quilt that will deal with the lowest temperature I'm likely to encounter, as I hate sleeping cold. The difference between a 2 season quilt and a 3 season quilt is only around 200 grams. The places I walk can always have a sudden cold snap so personally I'm not that fussed about the extra weight in warmer weather.
If 200g is a big deal for you, you could make a 2 season quilt for summer and a 3 season quilt for the elbow seasons. Then layer them in the winter for very cold conditions, with 2 quilts covering all your needs short of arctic or high altitude work. Just make one of the quilts large enough to drape over the other without draughts. For hard-core winter camping two quilts will be slightly heavier than a dedicated winter quilt, but they will be quicker to dry when you're dealing with condensation.
You'll find a lot more on YouTube and BackpackingLight.com if you search for "MYOG quilt". There are also kits on offer from a number of suppliers.
Hope this is of some use!