Looked for a letter of interpretation on this question from the Chief Counsel of the FAA but can't find one (I'd doubt there is one).
So, contacted my FAA Air Carrier Inspector buddy who confirmed my suspicion that with door open at the gate, seat belt sign is advisory prior to closing the door for departure. Once door is closed and aircraft pushes, then it's an enforceable regulation. If A/C is on taxiway, but stopped, seat belt is advisory again. A/C in motion is the key here.
If flight is delayed and door open, pax can leave aircraft but must remain in gate area
This is certainly the way I've seen it applied in my years of pax flying.
If there's an official FAA statement on this regarding at the gate, door open, etc...I'd like to see it. I don't know where else to look.