I'm guessing a small motor on the engine gearbox would be capable of turning the HP shaft, problem is it would have to turn the fuel pump, oil pumps, hydraulic pump(s) etc. This small motor is suddenly getting bigger.
I don't recall the size of the proposed rotisserie motor on the GE9X, but's quite small (yes, gearbox mounted, with a BIG gear reduction). It's only intended to turn the shaft at something less than 1 rpm so it doesn't take much torque.
More critical is the one way clutch - otherwise that little motor would be going a zillion rpm at redline N2..