I don't have XM, but I do have digital cable (Time Warner) at home with the music channels near the end.
There is a "big band" music channel which is pretty good for lindy, and there is the salsa/merengue channel for salsa. As for anything else, aside from country/western and perhaps some ethnic music/folk music channel, I can't think of there being anything strictly for ballroom there. It's still too much of a niche market I am guessing. Frankly an Argentine Tango channel may be more likely than a strictly ballroom channel.
But it would be a nice idea. I wouldn't have to play squat at dances I hold at home. Of course, whenever a bolero comes on...