No reason you can't have a custom receiver welded for the back of the trailer (tied into the trailer's frame) and then a custom rack for the outboard motor. If hitches can be fabbed to tow a boat or motorcycle trailer it certainly wouldn't be hard to fab up a means to carry the boat motor. It would probably be even easier for you to load and unload rather than from inside the truck bed!
I just had three (3) receiver tubes fabricated for the back of my trailer. They are all tied into the trailers steel I-Beam frame (although it looks like they are just welded to the factory bumper tube, they are not, they just pass through it). I carry a custom rack on each side for my generators and a can of fuel and the cent tube is for my bike rack.
It only takes imagination, a good fabricator/welder and money :)
