There are devices made for climbing trees in things like hunting blinds and whatnot. They are called tree steps (Google image it). They go into the tree at a right angle to the tree, then drop and go out again.Some of the tree steps claim 700#+ max.
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So the force of weight is applied downward, the dropping portion braces that force against the threaded portion screwed in.
I would not trust anything screwed into the tree long term as the wood surrounding the threads would surely die and decay, but on a large healthy tree, through and through, put a nut and washer on other side, and the tree should survive, eventually growing around and absorbing the head it would create, and the steps would remain mechanically sound. (Use non rusting materials!) Note: Only the cambium is alive in the sense you would expect, any other damage is just a wound where parasites, fungus, etc can intrude and decay the wood. Properly sealed the actual damage to the tree is very minimal. Think of old timber they cut into and find fence wire, nails, etc. once sealed in it is entombed, there are products to seal tree wounds just like when you cut a limb off. In fact some people have been known to "spike" trees, with very large nails in old growth timber, to wreak havoc on the would be harvesters of those trees, the trees themselves are damaged little to none.
You could fashion actual steps on top of them.
The real determining factor here would be the strength of the steel at the bottom 90, and the outward length.
IF you needed more support in theory you could do an angular brace at the lower end as well if you are building them yourself or having them built.
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But to be honest, I would not trust my life to it unless an engineer looked it over, and double checked my angles and material composition. :-)
Since this topic has my mind curious now, and someone was kind enough below to offer good suggestions to my engineering, it also occurred to me... If one were to fashion it out of angle iron rather than straight rods, put the wide end against the tree and turn the other upward toward the step, that would give twice the contact surface with the tree and step, and thus help with rotation. A good welder could cut and join that without the obvious brace as well, as the two sides of the angle iron would technically form that same mechanical structure twice over. The result would probably make that strong enough to lift an engine, much less hold a person.
Lets see if we can visualize that in ASCII!
Ummm, not so great...
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But you can get the idea. The angle iron would be stronger than a round support, and the way they would fit over one another when welded would form two of the angular braces while giving a straight 90 degree profile. Kind of making me want to fabricate one of these and test it.
