it would chain rustc and the ecosystem to LLVM
I don't think it has to. x86 assembly is specific to x86 architectures. Similarly, IR would be be specific to LLVM-based compilers. You would use a #[cfg] to make it conditional on IR support, just like you would use #[cfg] to make x86 assembly conditional on x86 architecture.
the IR format is not stable: a library may compile with LLVM 3.4 and 3.5, but not with 3.3 or 3.6.
I did not realize this. This makes me much, much more wary of the idea. Indeed, I think it's perhaps best not to support inline IR then.
I do wonder if there's something else out there that's similar in nature, but stable.