Forcing the cutout relay can risk exposing wiring to some rather extreme currents, not to mention messing with the precise settings of the delicate regulator. I generally replace the electromechanical regulator with a reliable solid state device anyway, so not an option for me.
I recommend disconnecting both leads from the dynamo, and then use a jumper to briefly touch the Field lead (smaller one) to the hot lead of the battery. One good spark is usually all it takes. You don’t even have to loosen the V-belt.