I have been very happy using one of Tom Seavey's Saddle Seats -- <
http://azlandtraditions.com/leather_Saddleseat.html >. I forget how much it cost, if I ever knew (it was a gift from my wife) -- but it is well-worth whatever it was. The heavy-duty leather is top rate, and Tom's workmanship is better than first class.
I use it in a 16 foot OT Ideal. Paddling solo facing "backwards" in the bow seat is awkward at best because of the location of a thwart right behind the bow seat in that canoe:
I place the Saddle Seat just ahead of the rear thwart:
which results in a well-trimmed canoe:
readily paddled solo.
As to using seats and seat rails as thwarts -- Bow seats are often hung with spacers so they are a few inches below the gunwales, to lower the center of gravity of the loaded canoe.
But seats hung in this manner provide virtually no structural support across the width of the canoe. When I got my 15’ 50 pound model built in 1931, the seat hangers were splayed out perhaps 1/2 inch because the gunwales had become wider as the canoe aged and the seat, hanging down from the gunwales, could do nothing to stop the spreading. If I had tried to hang the bow seat directly from the holes in the original gunwales, without the hanging spacers, the seat rails would have been a bit too short because of the widening of the canoe at that point.
Stern seats, however, are often hung with the front seat rail bolted directly to the gunwale. The extra height is supposedly allows the stern paddler to see better past the bow paddler (at the cost of a more tender canoe). Such a rail is, effectively, a structural thwart, but if it is lowered with spacers, it loses that structural role (which probably is not much needed that far back in the canoe).
The three main thwarts in the Atkinson Traveler above are quite adequate for structural purposes and need no help from seat rails (or the little carry thwarts). Seats, if you want them at all (and most of us do) should be located as a matter of taste/convenience for those paddling -- if a lot of tandem paddling, two seats traditionally located; if only solo paddling, one seat located for good solo trim. Our Ideal is most often paddled tandem, but does get a good deal of solo paddling -- so the traditional seat plan plus the movable/removable Saddle Seat works just fine for us. And when the 15’ is finally restored, the Saddle Seat can be moved from one canoe to the other.