I'm with Steve on this one. I use trusses when I am installing seats and they are very solid. There is absolutely no fore/aft movement like may occur with seats mounted with dowels. I think it is a good idea to leave a little space between the end of the seat and the side wall of the canoe so that dirt and water are not trapped there, but if side to side movement is a concern I suppose you could wedge a little cedar shim in there, and that should make it almost like another thwart, i.e., very solid. I say cedar since it is not supposed to rot. But I have never noticed any side to side motion in my seats and personally, I wouldn't worry about it.
I would be reluctant to mount cleats on the hull, but that's just me.
YakCatcher Rod Holder
Recreational Kayak Paddle