I don't know about this for sure, but I've been looking at seats on ebay (while I'm supposed to be working). I have pondered trying to retro-fit a seat without the stock sissy bar.
What I've come up with is that you'd have to have a machine shop make some custom bars to mount the turn signals to, and to keep from having holes in the side of the rear fender. Then it's just a matter of finding a seat that'll fit in the hole, so to speak. You'd probably have to drill a hole in the rear fender to attach the seat to also, since the sissy bar is where the bolts are that hold in the stock set up.
If you find an application that'll work, please post it here. I'm sure that there's be several who'd be interested (myself included)
I smile 'cause I don't know what the h*ll is going on.