i actually built a similar structure last year that seemed to work pretty well, and i'm planning to build something like that again this year.
last year, we had two pi-shapes made out of 3 2x4s each. they were leaning away from each other, but couldn't fall outward because they were attached with rope along the top. they couldn't fall inward because they were guyed down. Basically, it looked like this from the side:
/ \\ // \
The \\ and // are lumber, and the \ , / and _ are rope. I was worried about the feet slipping in, so I put some tent stakes down there, and then I draped a big 20'x20' canvas tarp over the whole thing. We had three hammocks hanging in that thing and it worked fine -- just keep all the lines taught. I would check on them in the morning when I woke up.
This year, we're building that in a 40' x 16' version. I found it easier to assemble by building tall saw horses -- only takes 2 extra 2x4s, it feels a lot sturdier, and it doesn't require people to hold up the frames while you tie up all the lines. We are going to build 8 saw horses total, 4 on one side and 4 on the other, about 12-16 feet between them. Enough room to hang lots of hammocks is the main objective.
Here's a photo: