The RX and servos can stand up to 7.0 volts (maybe a LITTLE more... not much). Note... that is not the same as using a 7.0 volt rated pack... that's about the peak voltage of a 6.0 volt nominal 5 cell NiCd pack. The circuits are designed for 6.0 volts from 4 cell AA alkalines.

You can get a voltage regulator to use the Li-poly cells without using a speed control and electric drive. Generally the ones you can buy are made for larger aircraft... and will not have been designed to be lightweight. The little 2 amp rated ESC's will be lighter, and you can put a dummy load on, then lock the throttle off. The ESC with BEC has the voltage step-down circuit in it. (small ones may only give enough power to the RX for 3 micro servos...)

There is a gadgets for RC website with plans for making your own voltage regulator. (it will be heavier than the ESC...)