Solar cells the size to fit on a r/c (as far as i know) won't be efficient enough to charge them while in use.
A good idea would be to setup a solar/trickle charger to keep your packs topped off on the way to the track, and while you are out bashing.
If you are out camping with no charge source, you could charge them all day. The would be ready for an evening bashing when you get back to camp maybe?