I get what the OP is asking. doesn't seem that complicated. if she can skip the front desk.. had used online checkin... and only has her back packs etc, with her. let's say she is staying at COronado springs. but has a lunch ADR at Captain's Grille at Yacht CLub. since there are not "assigned" Magical Express buses on arrival (like there are for departure) and her luggage is being transported to Coronado. what difference does it make if they take the Yacht club bus instead of the Coronado bus? none. it's not like they will be taking someone elses seat. planes get delayed. people eat lunch at the airport or shop. some even arrive a day early and stay at the airport hotel and take the ME the next morning.
then, of course, they know there is no resort to resort transportation.. so they will have to boat or walk to DHS after lunch and take the coronado bus from there. or take a cab. If disney doesn't allow this, I don't see why not.
as long as there is no carryon luggage (disneydoesn't allow luggage on the park/resort buses) OP, I would just ask at ME at the airport. "excuse me kind sir, we are staying at......... but would like to go to ........ for lunch . we have no luggage with us. is that a problem? and if so, why?"