You could probably even manage the horizontal beams. Do a web search
for Axel Erlandson and his Circus Trees. Here are two sites:

http://www.arborsmith.com/treecircus.html
http://www.bonfantegardens.com/trees/trees.html

M. Reed

Cloned Ranger wrote:

as i see it as long as there were no fully horizontal beams i could graft a
group of trees into a living treehouse...