There are several score (if not hundreds) out there doing this very job with MS Tamars all round the world.
P79 does not reduce the voltage from the one input i.e. if you want 7.2v output then you need a 7.2v battery in. The weight of a separate 7.2v Sub-C pack in this model should be no problem - the hull is a big as a baby bath.
As for the 3-pin lead, you just plug it into the receiver and it will autoset the neutral signal every time you switch on, so there's no need to muck about with flashing LEDs and Tx stick movements.
If you want a speed controller that will take in 11.1v and put out 7.2v then you need a P93 running on 50% ESC mode. That's as close as you'll get.
(Please no comments about my arithmetic - note the phrase "as close as you'll get" before hitting the Send key).
DM