On the lack of trees issue with hammocks.

Hennessey was showing a tripod assembly at NOAC 2006 (Boy Scout Order of the Arrow convention).

This setup involved two tripods with a ~10ft ridge pole connecting the peaks of the two tripods. The ridge pole and the tripods were constructed of 1inch (if I remember right) rigid metal conduit and a fitting for the tripod joint.

One hammock was suspended by two tripods and one ridge pole, with the fly strung over the ridge pole.

Two hammocks required three tripods and two ridge poles,
Three hammocks required three tripods and three ridge poles.
12 could be suspended using seven tripods and 12 ridge poles (hexagon with center tripod).

Not an system that I would want to take backpacking, but looked interesting for a base camp.