Reader Michael Alderete sent me a pointer to a blog post in which he details the process he uses to download high-def content via BitTorrent and than converts and transfers it to his TiVo for viewing, via his Mac. As he described it in his email:
The article covers installing and configuring TiVo Desktop software on your computer, downloading video from the Internet using Miro, converting it to a TiVo-compatible format with VisualHub, and transferring it to your TiVo for playback. Currently the instructions are Mac-only, but a reasonably savvy person could easily figure out how to use them on a PC, with slightly different software (pointers are given in the article).
Enjoy.