i tried to figure this problem out for a long time myself. if you were to put it behind the rear seat you would have to make some sort of mount to attach to the vehicle's frame. and you would have to have a hole in the tailgate to access the hook and cable. you could always go with the multi-mount system.
penguin in a cj has his winch mounted on a bracket that has a trailer hitch adapter build into it. He built it himself, and if it really is bad enough, he just gets a little dirty and moved the winch to the rear. Not the easiest or most "mall crawler" solution, but if need to use a winch in the back, the situation warrants getting a little dirty to save your rig. Atleast in my limited experience with eastern wheeling.
i could fab something like that out of 1/4 or 5/8 plate fairly easily. im going to be building a bumper for my jeep almost like that but i dont need the rear winch. it could easily be cut out of any aftermarket bumper. you would just need to reinforce the way that the bumper mounts to the frame or risk ripping your bumper rite off the frame.
if he has a tire carrier on his bumper that is a swing out i wouldnt get a longer bolt. think about the physics behind it. with 25lbs of steel and a 50lb tire hes carrying 75lbs on that bolt. by itself 75lbs isnt that much but once you hang it like in those carriers you have ALOT of torque on that bolt. if he puts a longer bolt he runs a big risk of breaking it.