This question comes up over and over again. The phrase "participating airlines" has caused all sorts on confusion with Disney CMs and with folks on this board.
Disney's Magical Express (ME) is not limited to "participating airlines." Passengers of
any airline can ride the ME coaches both ways. And when traveling to WDW, passengers of
any airline can check their luggage from their home airport directly to their WDW resort hotel. (It gets a little more complicated for international visitors who need to pass through U.S. Customs and Imigration at Orlando International.)
There is no such thing as a participating airlines list for the overall ME service.
The list of participating airlines only applies to an optional feature called Resort Airline Check-in. According to the Disney Meetings website
Disney's Magical Express FAQs:
Q: What is Resort Airline Check-in?
A: This service offers remote check-in, allowing you to check your luggage and receive your airline boarding pass at the Resort. At this time, remote check-in service is only available for guests flying domestically on the following airlines: American, Continental, Delta (including Song), and United (including Ted). Additional airlines may be added at a later date.
If you aren't on a Resort Airline Check-in participating airline, your luggage can travel back to the airport in the luggage hold of the ME motorcoach. Once at the airport, you can check your bags and get your boarding pass.
You can use Magical Express without using Resort Airline Check-in, and you can use Resort Airline Check-in without using Magical Express.