So someone put a 2s LiPo in one with no problems? I kinda figured it would handle it, but wiring in a voltage regulator would be safer, and easy to do. You never know how long the electronics in the radio will handle that extra voltage, but they will surely wear out sooner.
It's hard to say if it will handle it or not without trying it. Most electronics can handle a little more voltage than they are intended for, but it's never a sure thing. You could grab a small adjustable voltage regulator and install somewhere inside the transmitter. I am sure there is room for...