Does this topic have value for the OSE project or is it just your private interest?

A) To me the solution you ask about does not sound like an efficient long term solution because you convert chemical (gasoline) into mechanical energy then into hydraulic back to machanical and finally into electrical energy.
B) If you want electrical energy you could at least cut out the hydraulic energy conversion meaning to connect a generator to the combustion engine (as small add-on to combustion engines it is also called alternator)  or,
C) better yet, would be a fuel cell converting the chemical energy of gasoline or diesel directly into electrical.

The efficiency would be best in C) but this is not yet well proven technology. Then B) being state of the art. And last A) due to losses by unnecessary energy conversion steps.

The E in OSE stands for ecology meaning minimizing pollution and waste of resources...

The only advantage of A) would be that the presently existing interfaces (hydraulic plugs) of the power cube would be sufficient to hook up an external hydr.motor/generator. B) would require a new mechanical interface to hook up a generator requiring a design change of the power cube. But this might be worthwhile and increasing the versatility of the power cube. C) would require a completely new power module.

Depending on your requirements for the electricity properties like direct or alternating current (1 phase or 3 phase) and stability of AC frequency and voltage you would need additional electrical power converters (further reducing efficiency) or closed engine control loops to meet those req.