VESC as voltage regulator in hybrid UAV

Hi,

My project consist of hybrid powerplant for UAV, it's ICE generator with 4.4kW spinning BLDC  and 12S Lipo batteries. Currently I stuck with voltage regulation from generator, my main goal is to vary RPM on gas engine depending from load, but then I am getting variable voltage to charge batteries and for system supply. It's around 15-50V range. 
 

As far I know, VESC has regenerative braking, so my Idea was to use it. As bonus, it also can start engine, when running as motor. 

So question is: can VESC work in generator? And what efficency one can get from it?

Thanks