I think the most sensible solution is to use an AA portable 'phone' charger. Here's a link to an example, but I have no idea whether this charger will work with the N8xx.
http://www.amazon.com/Datexx-Power-B.../dp/B000M8UYDI

Why?

1) Lithium AA batteries are cheap and abundant. Check ebay to see what I mean.
2) AA batteries are small and easily transported.
3) AA Batteries are high capacity (Sanyo makes a mean 2700mAh NIMH AA)
4) AA chargers can also be small. Eg. An energizer charger I have folds to about the size of the N8xx, sans batteries
5) Portable phone chargers themselves are cheap and can be a DIY weekend project (for the more adventurous).
6) You can use the charger practically anywhere at any time, so you don't need to power down to replace a battery, or find a sunny patch.

}:^)~
YARR!

Sincerely, Corrupt