Rust doesn't have much of a community around it, besides Mozilla. Does that make it "proprietary"? Nope.
We all know NaCl is not going to be adopted-- not because it's not good enough, but because it's too good, and would threaten the native app ecosystems of Apple and Microsoft. pNaCl, same story. Google might end up using it as an app delivery mechanism in ChromeOS; that's about the limit of its potential usefulness.
It's particularly ironic to hear Brendan Eich complain about the lack of a standards-first approach in NaCl, since ECMAScript was designed behind closed doors at a single company. Anyway, ECMAScript seems to be good enough for building web UIs, and it's even a little less verbose than its "older brother" (whom it resembles not at all). So I think its quasi-monopoly is secure. I hope they pull the TypeScript extensions into the core language in the next version.