I discussed in another post about these homemade treehouse bolts I've made for a small treehouse and swingset. I've attached a few additional pictures.
I purchased twelve inch 3/4 lags from Boltdepot.com. I then put the lag through a six inch piece of galvanized threaded pipe with two floor flanges on either end of the pipe (with two or dock fender washers). Lowes seems to sell the galvanized pieces cheapest. I used a 3" forstner bit to drill into the tree and placed a hockey puck against the heartwood. I drilled a 3/4" hold through the middle of the hockey puck. I then pre-drilled and sunk the 12" lag through the pipe tight against the hockey puck, about 6 inches into the tree, with a 1/2" ratchet. I sprayed everything with LeakSeal for some additional corrosion resistance.
It works great as a perch and allows the trees to sway a little. I'm not sure of the technical shear strength, but so far everything is holding up great.