Hey Guys,
The RX voltage answer is to easy! Simply put one of the inexpensive ESC between the battery and the RX, leave the motor pigtail hanging! Make sure it is a Li Poly rated speed contro. This way you can move it over to your electric park flyer next weekend.

As to the TX most of them will handle 12 volts so plug in three cells.

But why the bother, if a NiMh AA pencell can give you 2400 mah why bother with Lithiums?? Simple charge using existing equipment, light weight, more reliable construction, etc etc.
JB