I just used Delta Skymiles to book a rental. Was told by both National and Delta that they could not attach my AE/ES to the confirmation. Called the rental location and they said that when I arrived to pick up the vehicle just go to the counter and they would make sure that it was attached. When I arrived yesterday, they said that it was already attached to the reservation and I headed to the ES aisle and drove off with no issues. So yes you are still eligible.