I've attended a meeting on the subject of providing audio/video documentation at the 21st Chaos Communication Congress. During that meeting, I was appointed as being responsible for this part of the 21C3 conference.
So we want to do on-the-fly encoding of four video signals from DC1394 cameras to DVD-compatible MPEG2, low-resolution MPEG4 for live-streaming, and OGG audio only for live streaming.
I did some preliminary experiments with the available experimental x86_64 assembly patches for ffmpeg, and it turns out that at least theoretically a 1.6GHz AMD64 should have enough power of doing those three encodings at the same time.
Unfortunately the dv1394 device at the moment only supports one encoder mmap() ing the ring buffer of incoming 1394 frames - but that should be fixed pretty easy.
I'll do some more experiments in the next couple of weeks, stay tuned.