In my AW setup menu, I can select mono or stereo sound. I don't have it in my mvs cab though, It's in my astro, so I haven't tried the stereo setting.
Of course, this doesn't necesserily mean that the stero comes from the Jamma connector (I would think that it wouldn't). It probably has to go through the IO.
The guy that I bought my MVS cab from just had a couple of quick disconnects on the 2 extra stereo wires. This works great, because I can just disconnect them if I want to plug in a standard jamma pcb.
I've heard of people just using select as the 5th button, but that seems extremely akward to me (and doesn't give you a 5th for p2)