I don't know how big the use case would be but it would be useful in some situations to have a decal placement modifier to dictate where in the stack any decals applied to the object take place.
Ive ran across a couple of situations where ive wanted to apply decals to the object prior to animation or geometry-altering modifiers and had to revert to using layered materials because the decal is only applied to the top of the stack.
One example could be applying decals to objects prior to applying physics simulations like cloth sims or destruction.
Thanks!