You may have already found this but there's a guy in the trailer porn thread (I think, if not then some other trailer post from last winter) that has done pretty much what you were talking about. I looked into it and you can get lift cylinders pretty cheap with various lengths of throw and lift weights so you might be able to "custom" size that way vs jerry rigging automotive applications with similar pricing...?...
FWIW, I've sidelined the idea because I don't want my boat sitting that high, I want it right over the wheels and doing so I'd only be left with about a foot of usable space... I'll probably just go with a side door or a hinged compartment in the middle of the deck and stuff firewood, spare oars, jackets and what not in there...
On another note, In researching materials for wood boat building I did a bit of research on pressure treated plywood and in general it's hard on fasteners and generally unnecessary. Normal CDX or better, has water proof glues and will hold up for years even in wet climates, is cheaper and doesn't corrode fasteners. You might rethink the pressure treated aspect...just my opinion.
I like the idea and wish you luck. Keep us posted on how things go!
"If you dont do it this year, you will be one year older when you do"