Step Turn,
Just out of curiosity, what makes the trip particularly challenging? Looking at the charts doesn't particularly highlight anything that demanding, or more so than a trip that distance anywhere in the world - but reading your response makes me believe that I am definitely missing something!
I understand that weather would be a factor to consider, but you would have that anywhere on such a long distance, the beauty of having a long trip and time to do it means that you can pick your route and days as you feel comfortable so as to alleviate the "stressful" alternative. It isn't a ferry flight that must arrive by a specific date!
At least in the UK, I have found quite a few owners who don't fly their aeroplanes as much as they would like to, and may be very interested in you renting their plane for a month or two at good rates, with a good number of hours to be done on their plane, especially just after the summer (Sept - Dec, not that this is any good for you wanting to go north....). A hangar queen is no good to anyone, a plane that flies, at least you know is able to fly! I am not sure schools / clubs are the best people to ask, I think you should try targeting owners and groups. I have seen a few websites where owners advertise their under-utilised planes - perhaps a few emails might help you out?
Good luck - sounds like an incredible trip!