I imagine they'd be stamped out very quickly, but just wondered if there's ever been any record of rebel groups trying to organise in DPRK?
Yes. In 1992, according to defectors who were in the North Korean military, there was a failed plot to assassinate Kim Il-sung, Kim Jung-il, and a bunch of other party officials at a parade.
In 1996, a military unit in the northeast killed their commanding officer and tried to start a revolution, but were shut down before they could get started.