Well, for starters, it isn't timber-framed. The joists are 3x8 and timber-screwed together...creating an inner circumference that is larger than the widest part of the tree trunk. The joists could be pre-assembled flat on the ground in 4 seperate sections... with a 2x4 temperarily securing the tops of the joists, in each section, in the precise orientation. Each section could be lifted into position, secured to the trunk with timber-screws and using a spacer block, and supported by using a couple of 2x4 as legs under the ends of the joists. Each section can be secured to each adjacent section around the trunk. Truts can then be installed to transfer the load...and the "legs" can be removed....or something like that. haha

Good luck,
Rooster