ncaissie the issue with a PVR style recorder is bandwidth. The input you need to use is just a controller input, which is USB1.1. Max bandwidth:11Mbps (iirc). You can make a webcam that sends compressed jpegs run on USB 1.1 with okay results at 320x240 or even 640x480 (but not that great I'd bet..). Unfortunately you wouldn't be able to do 24fps TV at decent quality.
A couple things:
1. you could do this with linux and any usb camera already. I know this isn't as cool of a solution as a dedicated app but I thought I'd mention.
2. Any homebrew software could also support other types of USB webcams, shouldn't be stuck to just the MS ones since as you said they are only available in Japan. It could be a good place to start, but actually.. there are other cameras who already have opensource drivers available, not to mention PC debugging apps to figure out how they work, so they might actually be easier to port than reverse-engineer an MS camera.