I think a surplus servo is a great idea. This makes way more sense to me then messing with F&P or axial flux generators. There are a Ton of these on e-bay for great prices if you look around. Do some homework on what electrical specs you need. I bought a medium sized fanuc motor last spring with intentions of building a turbine, but ended up getting really busy at work after and I havent built it yet. Also got a little discouraged by the lack of good blades available.
Your onto a great idea. My servo made decent power, I tested it in my lathe. I posted results of that on here if you want to do a search. Note that the one I had was chosen for its fairly high voltage output as I was planning on doing resistance heating only. This would not be a good choice for typical battery situation.