Currently, the purpose of R3S is to ease development, tests and so on. The "final user" isn't supposed to be "exposed" to this features
(or for very short moments for small files or because the package miss a few files, like it was the case in the ManiaDrive package for some Mandriva distribution)
The other point is that it works at filesystem level, it's not intrusive (there's no R3S related code in texture.c for instance) and, most of all, works for "everything" (textures, sounds, various conf/data files, PHP dependencies, ...)
So I don't see your idea as a tweak to R3S, but as a totally new feature. A rather interesting one, in facts. It's texture specific, probably a bit intrusive, but allows things like texture streaming (from a very low resolution to the final texture) and even better ...
But let's face it: it's probably quite tricky to write ! Forking/threading is probably needed, and the code should be bullet proof, since the engine should remains stable even when the download fails/stalls, and failure is something very common when you comes to network !