Mully,
Speedy here. I made some changes to my R3 Classic concerning the seats. I could not make the stock seat comfortable for my backside no matter what I did with the foam, short of adding a lot more. So I tried a Touring seat and found that it fit with just a few small changes and could be made comfortable by removing some foam near the rear of the seat. Then I had to add a passenger seat. Couldn't find what I liked so I made a quick change system so that I could ride "naked," with small bags only, or with passenger seat, seat backrest, and luggage rack. I made the seat and stainless steel brackets and reused the back rest and luggage rack. Mary loves the seat and the larger foot rests. The most expensive part of the whole process was having the passenger seat cover professionally stitched. The rest I did myself.
What I did may give you some ideas as to what can be done and how you might go about the changes. There are very few options available for passenger seats so you may be forced to adapt another brand seat to the Triumph bracket or build something unique.
See my photos for seat mods.