Welcome aboard to CD Freaks
There is a DVD-Fab equivalent for converting video files, which is called "VideoFab Converter". Going by its features list, it will convert the MP4 file format, so this should work with your exported TiVo MP4 files. The drawback however, is that it's a separate purchase to DVD-Fab, so if you are interested in it, I would suggest using its trial version to make sure it works first.
Another good alternative which I use for my own video versions is AviDeMux. Despite its name, it is a pretty good video conversion tool and has built-in resizing features which will let you reduce the video size for the Zen's native resolution, which will also significantly reduce the file size. This conversion utility is freeware (website link), so if it works, you're not out of pocket on another utility.
For my Zen (the older Vision 30GB model), I mainly convert to the XviD AVI format (single pass - quantizer at 4 and #B frames at 1) with MP3 audio.