I think you are all thinking about this wrong...He should have no issues with mounting a 10LB (combined) gas can and mounting bracket to the rear hatch. As long as there is no fuel in it, it should be light enough, and for bling purposes, it will never have fuel in it...
OK, just kidding. I have to vote no on a liftgate mount, because in my limited experience, I've seen too many accessories ripped free of the sheetmetal they are mounted to. This goes for fiberglass and other composites, as well.
A receiver mounted solution could work, if the "moment" is kept within the ratings of the hitch, and it is made to be very stable. By stable I mean it does not rock side to side or front to back at all. The hitch is probably rated for 350# of tongue weight, About 3-6 inches from the receiver. It is not rated for leverage from above, peeling the hitch away from the truck. Keep the weight as low and close as you can, overbuild the "drawbar" part, and install a rattle-free accessory. I have seen hitches fail and frames bend (once on a 22,000 lb chassis) due to improper loading, so I genuinely believe this is worth considering. Oh, these failures were on-road, "normal" driving. You know, "I was driving along with granny in the car and the darn thing just fell off."
03 Taco (Gone, and missed...)
88 FJ62 3FE, auto, etc. Sold
07 DL650, lightly farkled