@Moto Bike Mike, the reason hammocks suck in cold weather is that the cold comes through the bottom of your sleeping bag, since the insulation can’t do it’s job while it’s being crushed by your body weight. The way to fix this is to line your sleeping bag with a dense wool blanket (I use one of those Swiss army-surplus ones, very thin but very dense and heavy) before rolling out your hammock. I’ve used this method down to freezing temps so far with great success.
They also sell fancy inflatable hammock liners but those are expensive.
I don’t have experience boondocking at a trailhead (it sounds like an awesome idea though!) but I sleep in a hammock every night, I’ve anchored eye-bolts into my bedroom walls so I never have to sleep in a flat bed again. I haven’t experienced the knee discomfort that Head Over Handlebars described, which could have to do with the hanging angle of the hammock or simply the fact that everyone is different.
Anyway I’d say go for it! Unless the place has signs that forbid camping or overnight parking, it’s unlikely that you’ll have any trouble. I like the idea, I might try it myself this Summer.