Hi!
The basic problem is that the Data East hardware for Begas/Cobra/Road Blaster requires the laserdisc player to sync its video to the graphics output of the main PCB, instead of the other way around as most games do. The video output from the Raspberry Pi can't be synced to an external source, so Dexter is unable to support these games.
There is still
some hope, though. I have heard tell of a mod for these games that allows them to work with an LDP-1450 instead of the usual LDP-1000A. The 1450 doesn't have a sync input, so if the game can be changed to work with a 1450, it should work with Dexter as well.
Unfortunately, I have been trying for some time to get actual details about this mod, without success. I have a few theories about how it could be done, but I don't have complete Data East hardware, so I haven't been able to fully investigate this.
As a result, these games are still "unsupported".