I am not aware of any concrete plans to do anything like that. It is true that PNaCl generates LLVM bitcode, but so does clang, which is what Emscripten uses.
PNaCl has its own set of new APIs, none of which are supported by anything but PNaCl (and in particular not Emscripten); Emscripten does support the typical libraries a cross-platform app uses, like SDL, glut, egl, xlib. So it would be a lot of work to get Emscripten to do anything with PNaCl binaries given the different APIs.
Furthermore, converting PNaCl binaries to JS in the browser would add a lot of overhead compared to compiling to JS ahead of time and just sending the user the JS to directly run.
So if you want to run a C++ app in JS, you should just compile it to JS directly using Emscripten, PNaCl doesn't fit there in any way that I can see.
PNaCl has its own set of new APIs, none of which are supported by anything but PNaCl (and in particular not Emscripten); Emscripten does support the typical libraries a cross-platform app uses, like SDL, glut, egl, xlib. So it would be a lot of work to get Emscripten to do anything with PNaCl binaries given the different APIs.
Furthermore, converting PNaCl binaries to JS in the browser would add a lot of overhead compared to compiling to JS ahead of time and just sending the user the JS to directly run.
So if you want to run a C++ app in JS, you should just compile it to JS directly using Emscripten, PNaCl doesn't fit there in any way that I can see.