For this I used the MIDI input, one of the XLR inputs (for the vocal microphone), and one of the quarter-inch inputs (for the piano -- yes, I pointlessly mixed the fake piano sound from my keyboard with one of the fake piano sounds built into Garage Band).
So it seems to work. Now to read the manual to figure out how to make it dance and sing!
Chris is coming over to practice tonight, so if all goes well I will be able to do some nice recordings of that. Then hopefully this weekend (if not sooner) I can take a whack at recording the song of your choice -- you've still got a little while to vote if you haven't already!