@alevy To get an update done one has to be motivated to do it, and convince @alexcrichton at least that its worth the effort since he's usually the one dealing with all the fallout across the bots.
Since we're now trying to stay in sync with emscripten the way this will probably work is to do the Rust upgrade; test it on the dev machines against as many architectures as we can to be sure we've identified a viable commit; ask @kripken to prepare an emscripten upgrade; land the rust upgrade; land the emscripten upgrade. We want the window during which rust and emscripten are on mismatched LLVMs to be as small as possible.
It is not important for either the rust or emscripten LLVM to be based on a released LLVM, though it is important for rust to optionally support some released LLVM.
There is no defined process for upgrading LLVM - we do it when the motivation becomes great enough. I don't think there's been much demand for a more-defined process for updating LLVM but if you have some ideas about it perhaps you can say more.