Couldn't you do this already, with clever use of transparent textures and multiple layers of floor?
We're not talking about "maybe possible using some sort of cheap hack" but integrated, easy to use and supported properly.
Cheap Hacks = Fail
Hacks and complex tricks aren't solutions guys, if someone makes a suggestion, don't tell them how it can be done with such and such an engine trick, we don't need that.
It wouldn't work anyway. You'd have to have a bunch of textures just to have the transparency in the right places and it would take forever, and run slow. Not worth it.
[EDIT]
This is why I wanted it:
>image<
See those ugly seams? I was hoping to blur them.