Well, the lighter and smaller the better. That's where the battery pack option starts to fall down and why I started looking into dashcams for cars. Any idea what voltage/current a GoPro requires? Are there any purpose built extended battery packs for them that could be recharged in a car?
How limited are you by size/weight?
One of these (http://www.ebay.co.uk/itm/like/371093783490?limghlpsr=true&hlpv=2&ops=true&viphx=1&hlpht=true&lpid=108&device=c&adtype=pla&crdt=0&ff3=1&ff11=ICEP3.0.0-L&ff12=67&ff13=80&ff14=108&ff19=0) and a decent external battery pack should last you a good while in a go pro recording in a lower than HD res.