The trouble with doing this is that not everyone understands and/or follows the rules.
I was travelling one trip as a pax here in Australia some years ago when we (pax) were boarded during fuelling to speed up a turnround.
Most things were ready except for the fuelling, they had already taken away all the mobile stairs etc just the aerobridge left, and slides were not armed, when one of the flight attendants asked me to fasten my seat belt as the light was on.
I VERY POLITELY refused to, telling her it was illegal and offering to stand in the aerobridge even until fuelling was finished, and she said "we will see about that" and she stormed off to the cockpit to tell the Captain.
Next minute the seat belt light goes OFF, and there was a PA from the cockpit asking all pax to unfasten their safety belts.
Fuelling with pax on board is not a great idea, but IF you must do it, you really NEED to follow the rules.