Call DME at 866-599-0951 and tell them what you want to do and what time you want to be at MCO to pick up the rental car. They'll book you on a DME bus that will get you there around that time. All Disney resort guests are entitled to a roundtrip on DME, and it doesn't matter how you get to the airport or how you leave the airport. No flight required. This is standard advice on the Transportation board and is a very common procedure.
BTW even though you don't need a boarding pass for a flight - if you did, the Concierge desk at your resort could print one for you. Personally, we use the electronic boarding passes on our phones (add them to our wallets so it doesn't matter whether we have wifi or not).