So you're actually looking to discover how much torque can be developed by a turbo. I'm not sure how to determine that short of connecting a variable load on it. The first problem that comes to mind is any large generator is going to require quite a bit of power to develop much current for charging. Think of those small household generators that use a 10HP gas engine to supply 5000W of power. At 120V that's roughly 40A. I don't think a turbo will develop nearly that same amount of torque as a 10HP motor, and you will likely need a pretty good amount of currently to keep the battery stack replenished in a hybrid application.
What do you anticipate for the needed charge current? From an efficiencies standpoint you might be better off to do with a belt driven generator powered from a forced air drive engine.
-Rod