sponsorships and other stuff
You have a very good vision. The delima is:
(money raised for charity > money spent on the trip) That is, if you spend $8000 on your trip, but raise $4000, it doesn't really make sense.
I suggest doing the trip, planned out WAY in advance, and solicit sponsors for advertising. See if there are local organizations that would benefit from their name on a sponsors webpage. Write a book, or do a slideshow/dinner to raise money. You have to put months and months of effort into this to make it a worthwile event.
Maybe organizing dinner/slideshow presentations with local chapters of Amnesty internationl or whoever along the way.
Before you can legally say you're raising money for an organization, you must get official permission from said organization.
"They Shall Know Our Velocity"