My guess would be that this is impossible.
There is only one pin that’s free to use (D5). And the video signals must be generated following a very strict timing. If you take a look at the source code for the tv adapter you’ll see that almost everything is written in assembler. I think that the update-function of the Gamebuino library would compromise this code.
Furthermore the tv-out adapter code is using a lot of the available registers, so I think this would affect the Gamebuino library aswell.