Hi Edward
I have done this using my Rail-O-Matic milling system, I got the idea from reading an old book on Native American Indians, they used to take timber from trees without killing or felling them, they had a worked with nature policy not against it.
My system is easy to errect in around ten minutes, what you do is shimmy up the tree with a set of irons, cut out a gob at the height required and another at the bottom, lift the rail up against the tree and strap in in place a conversion plate which has a curved surfce and a ratchet strap, which wraps around the trunk, with the supplied winch, haul the saw and cradle to the top of the rail, swing the bar into the ready made gog or cut out, lock it in place and let gravity do the rest, I do not have any pictures of this but it worked very well.
you can see the ground version here below,
http://www.chainsawmills.co.uk/railomatic.htm
I love this forum
Davy.