If you want to put a LiPo pack in your TX, and you're worried about the 12.6V being too high, you can just put a diode in series with the positive lead (forward biassed, of course) to drop the voltage. I've heard of several guys putting them in w/o the diodes and not having a problem, but I remember seeing an advisory from one of the major manufacturers warning that they've seen radios returned with damage due to the higher voltage of the LiPo pack.

The other reason to put a diode in series with the positive lead of the pack is to block the external charge jack from the battery. If you forget you have a LiPo in there, and plug in the standard radio wall wart, you'll probably destroy your LiPo pack and radio! The diode will keep you from charging through the power lead, so you'll be forced to use the balancing connectors for charging.

I'm considering getting one or two of these TX packs from HobbyCity, and I'd put a diode in series to drop the voltage, just to be safe:

Rhino TX LiPo Pack

Zippy TX LiPo Pack

The cool thing about these packs, is they have a Futaba connector, JR/Spektrum connector, and a balancing connector, so you could use the same pack in just about any brand of radio. Unfortunately it looks like they are out of stock right now.