I had and idea and gave it a try. I bought slit cam straps that are 12 feet long. Look here to see what I mean: http://tinyurl.com/2r6bhd (this is a picture of a two-footer - they may still have some longer ones left.).
I put the short piece with buckle around the bowline I tied in the hammock rope and I put the long piece around the tree. The free end of the long piece goes down to and through the buckle near the hammock and can be adjusted by simply pulling on the webbing. I called the store and they gave me a rating of 500# for the buckles. This setup might not be light enough for some of you ultralight types, but for the backyard it is working for me. I have not crashed yet!
Of course, you could make your own split cam straps by cutting a long webbing strap and leaving the buckle attached to a short piece and sewing a loop in the ends of each.