WA1166 may be your best bet. You'll want overdrive to get the most of your power packs.
Any of the true 12V MR-16 and long life bulbs are unlikely to be very interesting driven on only 12 cells. At spec, they'll be pretty bright, but yellowish unless you go with a Solux or Ushio type color-corrected MR-16. And then, the pins wil be too large in diameter to fit in the MC's pin receivers.
You might want to just try some of the G4 bipin bulbs you can find at Home Depot. I think they will top out at 20-30W.
Wilkey