I asked that same question a while ago but no-one thought it would work too well. It may be ok just for the 12V accessory battery but not for the full pack.
There are plenty of (high priced) commercial solar roofs but I haven't heard from anyone that has one to know how well they work.
The other thing to consider is that if you want it for the whole pack, a single solar panel puts out too little voltage and needs an expensive charge controller to convert it. It's better to run 3 panels in series (each 12V) to get your 36V.
Here's the original discussion from my question:
http://www.buggiesgonewild.com/elect...olar-roof.html