Depends on efficiency of the gearbox. Tractor driven, so not a prioity probably.
540RPM is shown on the power graphs for the engine so no problems running at those speeds.
I have a 4kw (I think) unit which will be tagged onto the CS at some point. I would not expect more than a couple kW at tops, as the engine would be down to less than 4 1/2 H at that speed and the gearbox losses will be about the same (as it is still at rated speed).
In your instance a 12kW (or more) box will be losing a large % of your power, so I wouldn't expect much more than 1 1/2 kW, especially running a 10kW genny at 15%, what with cooling fan losses, excitation,etc with it being way off it's most efficient mode of operation.
Those boxes are often grease lubricated, BTW
In a nutshell - a waste of time in my view.
Regards, RAB