Hydrostatic
Glum's idea has merit. The hyd pumps are already onboard, and the plumbing is already in the nose gear well. A hydraulic motor is lighter and more compact than equivalent electric motor, especially if gear reduction is needed for the electric motor.
In other words, most of what's needed for hydrostatic nose drive is already onboard and used at other times, and used very little during ground taxi.
Not only do big tugs use hydrostatic drive, but small riding garden tractors as well.
GB