Keystone has a slide out type rear rack. Nash has a receiver and you can order their optional bike rack or tray. The frame on a Nash is built in-house and is off-road certified and a rear rack or carrier isn't likely to break off.
The KZ Sprees have a frame made from 3 pieces of 1/8" mild steel welded together and they flex a lot. I would not want a rack hanging off the back of a Spree because whatever you're carrying could be catapulted into orbit. We've had things bounce out of the sink in our rear kitchen, the stove grate jumps off and the open/close rod for the blinds falls off often.
Personally I would never use a rear rack because you can't see what's happening and won't know if something has been stolen, hit or flung off. There are carriers for above the tanks in the A-frame area. You might be able to get a carrier installed by a reputable frame shop with a certified welder on staff but you'll want a decent frame to start with. Adding shock absorbers to the TT would help.