Does anybody use an under the saddle bag for touring? (Not Pannier Bags)
I have seen similar setups for MTB touring that use a large seatpack, a frame pack and a handlebar pack to get enough capacity. This setup also allows a better weight balance.

Bikepacking.net

is a good site for getting ideas for different setups. This site might also give you some ideas as well. I know we aren't supposed to recommend gear, but I've been very happy with the stuff I have got from Revelate Designs. 

Revelate Designs

IMHO, your concerns about panniers are unfounded if you plan to tour on anything like a road. Panniers can be a problem if you are touring on trails or your bike geometry doesn't provide sufficient heel clearance ( often a problem on MTB bikes)