Given Sisko was still trying to contact Hudson as the other ranking Starfleet officer in the area when the Maquis declare themselves he at least still thinks Hudson was in Starfleet. Also Sisko tries to give Hudson his uniform back, which suggests that Hudson could just resume his position in Starfleet. Hard to do if he had resigned. Makes me wonder if Sisko had told Starfleet about Hudson either. Of course maybe it was purely symbolic. Given what Hudson had already done to that point whether he had resigned or not wouldn't have mattered. He would have been going to gaol.
As for Ro she was pretending to be kicked out of Starfleet when she went undercover with the Maquis. When she does decide to genuinely side with the Maquis it is played as an on the spot decision. She had no time for the formalities of leaving Starfleet. It was, boom, betray Picard, bye Riker tell Picard sorry, but not sorry. Ro out!