I have not found any commandline utilities that can write xmp data to a video file (exiftool doesn't support it).
I don't know of any, but that's not to say they don't exist. What specifically are you trying to do? Add metadata to a large group of files at once?
You didn't specify if you're on a Mac or PC, but if you're on a Mac, we're going to be adding the ability to import timecode and metadata into QuickTime movies from XMP data in the next major version (1.1) of our QT Edit tool in our Pro Media Tools suite:
Unfortunately I'm on windows. What I was looking for is adding timecode xmp start code to *.mov files from Canon 7d or 5d so I have timecode to use throughout Premiere which would make my editing much easier as we shoot approximately 100 video clips in the wedding ceremony and need to edit that in multicam w/ 2 other cameras running the entire time.
I did some more research and was able to add this metadata using xmp to the MOV files however unfortunately premiere completely ignores this data. Seems like premiere only accepts using timecode data for DV or MXF files and no others (tested CS5 and CS5.5). What a shame! It seems like this should be much easier thing to do. I know it's pretty easy using FCP and if Adobe is really trying to lure away FCP users, they should get something this basic running smoothly.