Yes Nick, tried a 3 inch long metal syringe tip as well as a 2 inch flexible one...same result, water bubbled right back out of the hole. I even tried loosening several of the other fittings associated with the bends in the radiator...no joy.
While talking about fittings...there is another knurled on on the side of the crankcase opposite the radiator...on removal you can see the crank and components normally accessible on the M90. I squirted a little 30 weight oil in there as best I could, though I expect the residual oil from the piston/cylinder above will drip on the crank and other bits.
Another learning...each knurled plug on the M91 has a center vent hole...convenient to fill fuel tank without actually removing that plug...I just use syringe with needle end.
Rog