Rudolf Cornelissen has updated his Matrox driver page with some good news for those who own those cards. Here's the update:

More good news indeed. We now have the VCD and DVD TVout playback modes for G400 upto and including G550 cards. I was indeed able to get them working nicely on G400 also now, although overscanning in NTSC VCD mode is very minimal. It seems the G400 MAVEN design has restrictions which are just a bit too narrow to support all important modes exactly as they are supposed to be. Note please that you will need the (updated) tool 'VideoScreen V6' for setting the VCD/DVD modes for now (see the techinfo and tools page).

So if you own such a card, download it and don't forget, report back to Rudolf if you encounter any problems, or just to praise him.