A wider seat is probably the solution. That 900 custom rear seat has one goal and one goal only- looks. It wasn't designed to be comfy!
Eyeballing it, it looks like the 900 Classic and Custom use the same fender rails. So, if that's the case, then the 900 Classic LT backrest should fit. You might need the entire assembly (including the sissy bar rails) in order for that to work though.
There are a few brands of quick release backrests. Why not nab one of those? It'll be cheaper than the OEM backrest anyway. Then, when your wife isn't going for a ride, you can pop the sissy bar right off! The seat comes on and off with a key super easy too so, if you really wanted to, you could swap between the mustang seat and the OEM seat for solo rides.
I've heard good things about the butt buddy, but the 900 OEM seat (custom or classic) is awful. Poor shape, poor construction, you name it. I really think, of comfort is a concern, an aftermarket seat is the way to go. Mustang, Corbin, Russel, etc. You could try a Kawasaki gel seat, but those that have used it don't seem to think it's really all that much better (some have liked it though).
Food for thought, but, I wasted a bunch of money trying to get my stock seat to work. Cushions, pads, airhawks, bead seat covers, you name it. Finally I just broke down and spend the big bucks on the Mustang seat. Was an excellent decision, and one I should have made long ago. It isn't as 'purty' I suppose, but it is very comfy. The one I have has a removable backrest. Though, it's never been removed!
"8 But God demonstrates his own love for us in this: While we were still sinners, Christ died for us."
Romans 5:8 (NIV)
2014 Kawasaki Vulcan 1700 Vaquero ABS SE
iPod Connector Kit, Kuryakyn Highway Pegs, Mustang Touring Seat, Marvella's Hitch, Kuryakyn Trailer Wiring Kit, Haul-Master Tag-a-Long Cargo Trailer
2011 Honda Shadow Aero 750 (Wife's)
Memphis shades quick-release windshield, OEM Solo Seat, Mustang Fender Bib, Chrome Solo Luggage Rack
Past: 2006 Vulcan 900 Classic LT