How about getting a big rear chromed luggage rack that spans the rear fender, leave that installed. Then you can get a rear touring bag to use for trips when needed?
I got a Tour Master
, you can unstrap it and bring it inside a hotel. All sorts of sizes.
Mine is small, says it holds 10lbs. But it came with the bike. I'm sure they make bigger ones with a 'back rest' cause you do need it to strap the bags in. I mean, it'd be no different than just putting your existing seat/sissybar on. BUT, not sure what you want. If you just prefer the look of no rear seat, and if you wouldn't mind spending the money for a big rack, and wouldn't mind having that on permanently. I'll never have a passenger, and wouldn't mind removing my rear seat/sissybar and the small rack, and just getting one big one so I can attach my bag when needed. But that's a one day project, needless accessory. Cause why buy a rack and do the work, when the rear seat/sissybar accomplishes the same thing (holding my bag). Just something to piddle with and change for the hell of it and waste money I guess.
(BTW I don't put my big bag on that little rack, the bag empty is 10lbs) Just posted it for a visual.
And after googling the Saddlemen BR1800, ya, I wouldn't use it, or my Tour master or any of the those luggage types w/o a backrest. Mine has other straps aside from the backrest strap, but they're for extra stability (mines a big arse bag). OR... get some cloth, cut to size, cut holes for mounts, then mount your seat? I can't remember if the fender under my rear seat on my 650 was any kind of scratched or worn. Haven't taken the rear seat off my 950 yet.
I dunno, just some ideas. Rambling cause its morning time and I have nothing else to do