NaCl does not allow direct manipulation of the DOM or provide other JavaScript APIs like canvas. NaCl provides alternative ways of getting things done like pp::Graphics2D, but it is not meant for DHTML. Dash, on the other hand, provides access to JavaScript APIs.

I think that browser "assembly" code, like Dash, would have to support this, particularly if it is meant to be an alternative to compiling to JavaScript.

PNaCl seems like a separate project from NaCl to me, and NaCl itself can't run LLVM IL.

Also, there's really opposition of having it implemented in other browsers, at least Mozilla vocally opposed it. I think that if they managed to do it via a plugin though, that could work.