I don't know of a ready-made single seat conversion for the XBR, but I have seen it done, bespoke fashion. A local restorer got an upholsterer to re-foam the seats of a couple of his XBRs, to create a hump at the back, following the lines of the rear seat cover but coming up a little higher. Looked pretty good. I don't have pictures, unfortunately. If you draw what you want and take it to an upholsterer who does bike seats, it should cost you a lot less than an after-market seat, were one available.
If you could find one, you could probably bodge a GB500 seat on, but the longer XBR frame rails would poke out of the back ;-)
