I've enjoyed reading everybody's ideas but in the end you have to decide what you want to do. if you stick with the idea of strapping down the back, i would suggest that you approach it this way. Only strap down the back that's where the cage is the highest. place your strap as far back along the rear trailing arm as feasible. the guy who suggested the climbing bolt hanger gave you a good suggestion but i wouldn't put it on the shock bolt. On the rear trailing arm there is a 3/8" bolt hole below the black plastic brake line clamp. i would bolt a hanger (or a piece of angle iron with two holes in it) here.
I would then make a loop of 1" tie down strap about 4" in diameter and loop it over the tube under the bed in such a way as to have both ends meet giving you a place for a hook and when it is unhooked it can be removed easily. these are the two locations that i would place a 1" tie down strap the winch type.
these tie points will keep you from placing any side stress on your shocks and will put you into a location where access will be easy and will give adequate leverage so that winching will also be easy.
Once you get your RZR into the trailer, rig your transport tie down setup so that you pull the back of the RZR down, further compressing the shocks. this will take the pressure off of the straps that you used to give the vehicle more squat.
Neatness counts, keep your lines straight, keep your angles equal, protect paint from scratching and metal from bending. by pulling your RZR down compressing the suspension further when it is strapped down for transport you will be putting the forces in the strongest places, i.e. main framing of your RZR and the trailer floor. (this is only true if the tie points in your trailer floor are actually structurally sound)
concerning wearing out or breaking springs, Your pickup has been setting on the same springs for its whole life. so has your trailer, your motorcycle, the loaded semi-truck on the freeway. I leave my RZR in the trailer strapped down for weeks on end. when i was motorcycle racing my bikes were strapped down in my van all week long and only came out for maintenance, riding or racing. Don't worry about the springs.