Hi David.
I was just writing a post now, trying to explain too why offscreen rendering won't help us on this case, and got your new mail while writing it, so here I start again... ;-)
I think your new suggestion is quite interesting, it sure is more general than BTF / FTB, although I personally don't see any useful painting orders except BTF / FTB (of course, the fact that I personally can't find any doesn't mean there aren't any).
I mostly prefer the more generic solutions, and this is why I prefer your way over mine, although implementing 3d in this way will really be harder.
I volunteer for trying to implement your interface and making both 3d and cube use this interface if you are interested.
What do you think of all the other patches? Do they look okay?
Thanks,
Roi.