Good stuff!
Re: "Bring Emscripten (and its PNaCl passes) over to LLVM 3.6. This is all a hack that just happens to work, and it'd be much nicer if we didn't have to worry about the incompatibilities between the two versions."
We've had a few patches recently to improve Rust support on Emscripten/PNaCl. PNaCl is close to tip-of-tree and will be past 3.6 soon, and I expect Emscripten to follow. The bitcode version issues shouldn't matter if the Rust frontend is built with the Emscripten/PNaCl codebase, which I would say is a medium-term goal.
There's a GSoC project to improve this:
https://developer.chrome.com/native-client/reference/ideas#rust