The most sustainable type of guitar is one made from completely synthetic materials, IMO. They just haven't caught on, yet... I'd like to see the phenolic Reverend guitars come back, but that ain't gonna happen any time soon.

One of the most interesting ideas was the Lamboo Telecaster, one that they made for the 60th Telebration using bamboo laminate. I think it's a great idea, as bamboo grows fast and is sustainable as a building material. Reports that I've read about the guitar are favorable.

As far as amph construction, I'm largely ignorant, and don't feel like I can contribute much. I do wonder, though, if fiberglass, carbon fiber or some other material can be used for head and cab construction and stil sound good.