You Sir, are a clever man 
You're describing phase 2 of my plan to a tee. 
I can't speak for other covers, but my FoldACover will hold 600lbs easily (tested it myself). I plan to add cleats that line up with the front and middle feet on the cot, and extended bars that mount to the lowered tailgate. If done right, the cot won't move an inch.
Here's some very rough proof of concept pics I just ran out and took. Please note the cot isn't fully set up; with the end support bars installed it is tight as a drum.
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[ATTACH=full]190314[/ATTACH]
I'm not sure when or really even if I'll get there though. The cot on the ground right up against the truck has worked out so well so far I don't necessarily feel the need to put it up high normally. I might end up making it more of a priority after spending a night or two in bear country. Not sure.
But either way, you and I are on the same page, and I picked this cot with the idea of going up top in mind.
PS: For size perspective, here's the cot, tent and pad all wrapped up in the bag that comes with the cot:
[ATTACH=full]190315[/ATTACH]
I can go from this bag, to fully set up in 5 minutes or so without hurrying.
