Not sure how useful this can really be given that most modern desktop OpenGL apps use various APIs and features that are not implemented in GLES/WebGL.
Likewise, while emacripten is a neat hack, there are a great many limitations imposed by JavaScript that hampers the ability to port a lot of interesting C/C++ code, not to mention the massive overhead compared even to well-written native JS. (I gave a talk a few months back on optimizing JS game engines, and needless to say it requires a great deal of hand tweaking and low-level knowledge of the language that retargetting compilers abstract away from the programmer.
A better solution would be to just get Firefox to support NativeClient. Especially as Unity 3D (an another two major game engines I can't disclose atm) are going to support NaCl as a target in the next year or so. Funnily enough, they're using that to claim Linux support without any "native" Linux binaries.
