Clumping bamboo is, well, clumping bamboo. Its characteristic is to grow in a clump...always. For this reason, if you want to create a screen you need a fair number of them planted fairly closely together since it's not going to fill in gaps very quickly because it's...clumping. As for strength, there is probably nothing you can grow -- clumping or running -- in your zone that will be big enough to support a hammock without getting pulled over. Lastly, I've never heard of a permit required unless you are in an area with restrictive homeowner association laws. I suppose you could always get somebody complaining about view restriction, as with tall trees, but I think it highly unlikely that your municipality has any regulations on bamboo. Good luck.

Most of the timber bamboos will "freeze" below 40 degrees or so. But a 20 foot bamboo is not a timber bamboo.
There are some bamboos that will thrive in your zone. If you have a start that is doing well, and staying put like you want it to - you are well ahead of the game.

New England Bamboo has a pretty good website.

sunbeam-
I'm not clear on your message. There are many running and clumping bamboos that are very hardy down to 0 degrees F, and some lower. I'm in Zone 8, like blewb, and I have over 60 different bamboo species in cultivation, including a number of timber bamboo as well as clumpers. Last winter we got down to about 15 degrees F several times and I didn't lose a single bamboo. In fact most of them didn't even suffer any foliage damage.

Kudzu:
Since you're in the same zone as I do you have any advice on a good clumper that won't try to spread out too much? I'd like fairly thick staves as well but, if possible, somewhat short (10-15 feet). Anything like that?