If this could encode an MPEG in realtime (it should be fast enough on Desktop PCs and browsers with asm.js support, right?), you could send WebCam video via WebSockets to a server, distribute it and decode again with jsmpeg[1]. Sort of like the ghetto version of WebRTC, but without all the codec hassles :)
It's single-threaded and asm.js still has overhead, so I doubt you're going to approach the performance you get from ffmpeg compiled natively for your machine. I expect native ffmpeg uses SIMD, too.
If this could encode an MPEG in realtime (it should be fast enough on Desktop PCs and browsers with asm.js support, right?), you could send WebCam video via WebSockets to a server, distribute it and decode again with jsmpeg[1]. Sort of like the ghetto version of WebRTC, but without all the codec hassles :)
[1] https://github.com/phoboslab/jsmpeg