Looks like the TiVo 7.2 beta is over, as Tivo Desktop version 2.2 was released today. Coupled with the newly released Tivo OS 7.2, transfering video from a PC to your TiVo is now possible. The same earlier referenced TiVo support article explains the functionality and how to convert your other formats into the proper mpeg2 version.
A reader that has the new OS and desktop software writes:
Your PC with Tivo Desktop shows up as a server on your Tivo's now playing list, and any mpegs in your "Tivo Recordings" Folder show up on the Tivo.
If anyone has the new OS and software, can you leave a comment on whether or not streaming video from the PC to the TiVo is possible? It'd be a shame if you had to move half a gig of video files over before you could play them. If streaming is possible, this would allow you to use your PC as a video archive, by storing video there instead of the TiVo hard drive.