Maybe make it illusionary and make the decal appear using env render? (i don't even know if you can use decals/renders that way, but maybe try it)
You could simulate by making a shootable button with custom "switchable" texture (the +A texture would have no decal, and the +0 would have it when you shot it). There would still be the problem of the bullet going through, since the bullet won't activate the switchable texture AND hit the thing on the other side (afaik), so you'll have to use triggers for the latter i guess.
Tommy14 texture types