I'm also interested in using a servo motor as a generator.  Not for a windmill, but on a waterwheel/hydro system. I'm no servo motor expert nor do I have a good understanding of inverters and battery charging systems. Grid tying is out of my area of interest.
However I have "played" with a number of different servo motors and I appreciate their unique "generator like" nature.
It was a long time ago that I discussed this with Zubbly, who at first had doubts.
But after chatting with him and doing a few "very basic" tests and reporting back to him his doubts were waning.  Sad to say I was unable to do further testing before Zubbly left us (damn, never really got to know him and still miss him like hell)

Servos are very "torquie".  What I mean is even though you may get a low voltage out at a slow speed, they can produce lots and lots of current.

When Servo Motors are used AS servo motors they can produce their power ratings (lets say horse power) even at low speeds (they just use Gobs of current at a low voltage).

Most everyone will point out that to get the needed voltage you'll need to "gear up" your drive system.   And everyone will agree (me too) that gears pullies and belts will rob power and be a maintenance nightmare.

With a "current" generator there are ways of addressing the low voltage issue.
My idea was to run the output of the servo (their 3 phase) into a 3 phase step up transformer.  Yes, they also will rob some power but maintenance wise.. no gears pullies bearings  belts ect.. ect..

Before you abandon all hope I'd suggest you "play" a bit more.  All the servos I've played with have one or two bulk connectors on the end bell.  You already located at least two of the power connections, the third will be right next to them.  
Also note that you don't need to use a 3 phase step up transformer.. you could use three separate trannies.

There is also the possibility that you can "break into" the wiring of the motor to get a "higher" voltage.

I'd suggest taking the power connector of the motor off and checking out the way it's wired (the number of leads going to the three power pins)

If your interested take a peak at my files and look for the KOLLMORGAN pics.
I think they are pretty big files so beware.

and Like Zubbly always said