I've got one of these:-
Portable Power Supplies - Portapow USB BatteryCharge it up (by mains or computer) and carry it with you in a little tri-bag on the top tube:
http://www.greenbank.org/misc/IMG_0909.JPGI used mine on a recent 400 where I took nearly all the time available (26.5h).
7am start, GPS fully charged, no power pack attached
8pm, at the control I connected the power pack and it recharged the GPS whilst powering it, including powering the backlight of the screen for the night stage. I left it on all the way to the finish (9.30am).
Portapow pack sits in the top tube bag with cable dangling out connected to the 705. The retractable cable means that you can put in enough slack for steering not to be a problem, but no chance of the cable dangling down towards the wheels.
At the end the Edge 705 was completely charged, and the unit was still showing 40% charge (2/5 lights). So it would easily have carried on another 13 hours to finish a 600.
The cable to the Edge 705 is also wired in such a way that it doesn't trigger the Edge 705 to go into computer mode, so there's no worry about it doing weird stuff when it is plugged in.
I also bought one of the 2xAAA battery thingies they sell, great as a backup should something go wrong as you can charge up the Edge with batteries purchased along the way.
It also came with the right connector to charge my iPhone, which is a bonus.
It's not waterproof, but there's no reason why you couldn't put it in a ziplock bag in the top tube bag. The connection at the GPS won't be waterproof but I'm not worrying about that too much, it's a pretty snug fit and I could always tape it up with electrical tape to keep the worst out.
Being LiIon it's pretty light too (120g).