RMC,
I can't imagine 14 C.F.R. 121.317(f) applies in the situation you describe. We were required to enforce compliance if the sign was on and the airplane was moving on the ground (push back/tow in//taxi) or when belts were necessary in flight. If the main cabin door was open and the jetway attached, you could likely get off if you wanted. The airplane isn't a jail. Just tell the F/A you're stepping out...and be prepared to get left behind if you aren't back on board when push back starts.
Our procedures called for turning on the sign during initial cockpit set up and turning off when appropriate in flight ( with a PA admonition that even though it's off, it's prudent to leave it fastened).
If anyone was ever prosecuted and fined by the FAA for not being belted at the gate with the main cabin door open, I'd like to read the story.