Most Western ski areas are located on national forest land, so choose your campspot properly and you won't have to worry about being on someone's private property.
There are limitless options for camping around Mt. Hood in the summertime. Just pick any forest road and drive until you see a spot you like. You're legally allowed to camp for two weeks in one place. If you prefer company, there are always snow gypsy camps that you can join.
At Hood you won't be able to walk right to the lifts, because the camping is done lower down on the mountain (around 2500-4000' elevation) and the resort is up the mountain about a 6 mile drive and 2000' feet higher. Theoretically you could camp up near the lifts, but it gets brutally cold up there at night — you probably wouldn't last longer than a night or two. What you could do is camp down lower on the mountain, near Government Camp, and hitch a ride up Timberline Road each morning - it's easy to do. (Pro tip: West Leg Road above Gov't Camp for easy, walkable access to T-Line Road).
I have no experience sleeping in hammocks, but I've heard that you stay warmer in a tent on the ground with a good sleeping pad. Bring the 0 degree bag, it's better to be too warm and have to open your zipper than be cold and have no options.
My experience is based on Mt. Hood, but I would imagine the other resorts you mentioned have similar situations. Don't plan on being able to walk to the lifts; you'll most likely need to drive or hitch.