There is an apkjs IRC channel on mozilla [1] so they might cross-compile Android apps via emscripten to asm.js. Couldn't get any info from moz when I asked around in that channel.

[1] http://search.mibbit.com/networks/Mozilla/%23apkjs

Could be pretty cool if they allowed Rust apps.

That's part of their agenda. If they will let people easily use other languages, naturally only JS devs will use JS.