If you get a small panel (7W maybe?), I hear you can live with the panel having an open voltage higher than what the radio accepts - the current draw from the radio causes internal resistance which lowers the voltage that the radio sees.
The FT-817ND is pretty forgiving with input voltage, but connecting a higher wattage panel directly is not a good idea. You will need either a solar charge controller for a sealed lead acid battery (avoid MPPT types, they tend to be RF noisy, heavy and expensive - use a simple resistive type), or something like a LiPo laptop/netbook battery pack. The latter will accept a direct connection to a solar panel, and charge an internal LiPo battery which again will charge a variation of netbooks/laptops, 12 volt, 9 volt and USB (5 volt) devices.
Here's one such battery pack (Energizer XP8000) used with the FT-817ND - looks pretty neat: http://www.youtube.com/watch?v=IsvPOyn0LeQ
A list of possible candidates:
- Energizer XP8000
- maybe Energizer XP1800 <- Not sure if the voltage regulation on this will work with the FT-817ND
Tom's Hardware has a thorough review of these models here: http://www.tomshardware.com/reviews/notebook-battery-external-power-supply,2821-9.html
Those backpacks are only for USB charging, it seems - too low voltage for the radio.
Not only charging the FT-817nd, I would also like to charge my cell phone and mp3 player)
Then one of those LiPo battery packs seems like an ideal solution, given that they have both a USB port and many other outputs running off the same battery. You strap the panel to your backpack, and connect it to the LiPo pack, without having to have it connected to the radio all the time. Also you could bring some spare internal NiMh packs. (If you want to play around with internal LiPo and LiIon packs that is a possibility if you understand the dangers involved).