There can be an issue about heel strike. I run 10.5 to 11 feet on a 6'1" frame that doesn't seem to be a heel strike fiesta, and I used to tour on MTBs with special MTB paniers (smaller, sorta the alpinist pack vibe vs backpacker gear was the trend when I bought them). At that time most MTBs had rack mounts on them but suspension did bypass that to some extent. You would just have to give it a try. There are ways of moving the load back, but they aren't all that clean.
As far as the gravel road thing goes, MTBs aren't really special as far a roaded terrain goes. People drove bikes with big tires, gears, etc... offroad from the very beginning. If anything 700c will float more, and 26" wheel touring bikes will be indistinguishablefrom MTB where the rubber hits the road. You don't need 2" plus tires for that kind of thing for the most part, 1.5" tires will do most anything. The real advantage to MTBS is cost and availability. Use what you have, or can buy for next to nothing. But there is a "proper" touring bike for anything you can imagine, short of snow and 4" tires in the desert.
Check out the Riv site to see slide shows of people riding their touring bikes (and some MTB like bikes) on gravel roads.
http://www.rivbike.com//