The official Fort policy is nothing tied to trees. Do they enforce it, No for most of the time. With that said about a year ago there was some guests in the 1300 loop that set up a zip line in the center of the loop, very high in the trees and about 150' long. They (little kids and adults) used a ladder to climb up and take off from the highest point (30'), it took about 5 minutes for someone to call the FD and report it and about 3 minutes for Security to tell them to remove it. I have seen many hammocks up so I think you will be Okay. Have fun at the Fort.....