Your main concern here is going to be shock loading. Nuts are typically tested to resist static compression load. Any shock is typically adsorbed by the rope and other equipment, so they get the mimimum shock load when/if you fall. If they don't fit well and are loose (or are just passed though a hole and are therefore moving around all over the place) then when/if you fall they will hit the rock and shock load. There is no give in the wire or metal so the full fall factor is concentrated in a very small area. This has the potential to take the equipment past it's limits and therefore break it.
Even ignoring the shock loading on the equipment your also going to shock load the rock, this can make it break. Think pulling on a hold vs hitting it with a hammer.
Then there's your slings, etc. Depending on the material you are again potentially shock loading them due to the movement in the anchor.
All in all, this is likely less secure/safe than a well placed secure nut in a crack and should be avoided.
You'd be better off attaching a nylon sling around the "tunnel".
Here's some work DMM did on the effect of shock loading on slings to give you an idea of how it can affect a system.
Or would the forces even be smaller since the stopper distributes them
around the hole
If anything you're distributing the load less evenly. A well placed nut should have the maximum surface area in contact with the rock. The more surface area the more the load is distributed. Only the top of your knut is in contact with the rock and if it's loose then not even that initially. So your actually concentrating the load!
What I'm trying to say is this:
Rope tight (looks ok?)
Eveything seems ok in the above scenario
Rope loose (not so good now)
Nut no longer in contact with the rock
C---|| ----->rope goes taught (moves right)
-> | \
Nut impacts with rock!!