Be advised that this little project takes two liberties (easily changed by forking and recompiling):
It is hardcoded to use the Zero Latency (real-time) setting because that is what I needed.
It changes the four-cc code from 'avc1' to 'x264' in order to avoid a collision with the built-in OS X H.264 encoder.
Another caveat: this build is Intel-only, and I have only tested it on Snow Leopard.
A binary download is available here.
To use, simply un-zip and move the x264Codec.component to /Library/QuickTime (This may require administrator permissions.) (Or you should be able to use your user's Library/QuickTime folder.)