I recently got a used Bell Morningstar with a sliding bow seat (much like the Stewart River pic that Glenn posted) and a center seat for solo paddling as in the Eaglet above. I tried it both from the center seat and from the bow seat backwards for the first time yesterday.
The issue with the center seat is that the boat is so much wider there. While the trim is improved, it's harder to keep a vertical stroke with a single blade. With a double, you'd need it to be that much longer and you'd be doing even more of a sweep stroke. Heeling the boat over with a single blade would help this.
For the bow seat, I was pleased to find that I could slide the bow seat partway towards the end of the boat and have room to fit my legs in between the seat and the thwart that holds the end of the seat rails. You gain several inches advantage in trim by doing so, moving towards the center of the boat. There's a trade-off, though, between getting better trim and pinning your legs between the front of the seat and the thwart. One could make longer rails and a wider thwart to hold them, moving the whole rig towards the center of the boat. But, you'd be dealing with increasing levels of complexity. Is the boat ever used tandem, or only solo? Getting a center seat and replacing the bow and stern seats with thwarts might be your answer if you only solo it.