That's annoying. I suspect the R4 will stand up to most brutal approaches that might be used to release a clutch. One possibility is putting it on axle stands, starting it in gear, then with the cluck pedal depressed playing the brakes against the throttle to try and free the clutch.
Otherwise there is a hole in the top of the gearbox used to check the timing. Might be possible to take advantage of that if you get desperate(and with the engine turned off), although a screwdriver would probably cause damage to the clutch