The solar rolls are *NOT* that robust. They can't be folded or rolled too tightly. Even the small ones take up A LOT of space in a backpack.
Couple that with contacts that WILL wear out with the constant rolling/unrolling potential and the weight issue -I wouldn't touch these cells.

There are far more effecient cells out there (weight and size wise). I'd purchase a solar battery charger with Nickle Metal Hydride batteries. I too was hoping for something compact from freeplay but nothing has come out yet.

Perhaps you can make your own:

http://www.geocities.com/cameron_goffin/swsolar.html?1013539610790

-NIM