The manufacturer had to choose the nominal voltage for the specification. I'm not sure exactly how they do that, but it would be at some specified current load. The rotational speed - rpm - required to hit that power output has to fall within the mechanical specifications. At some point, I think higher rpm no longer gives as much power as it should, due to inductance.
Running at higher voltage but lower load shouldn't be a problem as long as you're not over rpm or overload.
Typically most things like that still have to pass a HiPot test which can be anywhere from 500 to around 2500 volts from windings to case so over voltage working as a generators is not an issue.
Odds are the rotor would fly apart well before the output got anywhere close to the 500 volt bottom end rating.
As long as you are not exceeding the designed and rated current for extended periods of time you can pretty mush spin a motor up as a generator until it flies apart.