Merlin,
I do 99% of my carvings outdoors and have done many of them 25 to 30 ft up. It is not a problem as long as you dont mind heights and are super safe. What I do is rent scafford from the local rentall store. A section of scaffold cost me about $17.00 Canadian for a weeks rentall for a carving that high you will probably need 3 sections of scaffold. Dont try and put up that much scaffold yourself it could be hazardous to your health. If you are working that high make sure you wear a safety harness, a fall from that high would kill you. Have you considered carving the eagle from the existing tree? The only problem with this is that you have to keep moving the scaffold. As far as how it will weather will depend on what finish you use and how often it is reapplied. I use Sikkens stain Cetol #1 for the primer coat and Cetol #23 for the second and third coat. This works for me and I advise the customer to reapply it every two or three years.
Hope this helps if you need any more info email me I would be pleased to help you any way I can.
Colin