4 AAs is about 6v, 2S lipo is 8.4v and a 1S is 4.2v. Think thats the reason you haven't seen a lipo tx batt for these, voltage isn't right. LiFe is even a little too much at 7.2v.

I'm using 4 AA LSD (low self discharge) rechargeables and like how they work. With these NiMhs they practically don't lose voltage just sitting and you only lose voltage when you're using the radio.