[MetalMan]

It is very possible, but not so simple. The React as you know uses a 4 cell NiMh battery, which is 4.8v nominal. The React is good for a little over 6v, but much more will fry the USB functionality of it (at least 2s Lipo did...). So what you need is a 5v regulator (5v UBEC works great), which will allow you to use any Lipo pack you want, I am using a 2s but 3s would be fine. However, since the Lipo pack wouldn't be connected directly to the React you won't know the voltage it is at, so there is a chance for overdischarge. To solve this I implemented a LVA circuit (audible low-voltage alarm) that kicks in when the 2s Lipo is a bit over 6v. It's a noisy little bugger so I know I have to charge the Lipo as soon as I hear it.

The complexity of this Lipo "conversion" is likely the reason why you haven't seen/heard of too many doing it.