Yes, Magical Express is FULLY capable of providing DME transportation to Guests in wheelchairs. You simply need to let them know in advance that you will need a bus that can accommodate your chair.
And to expand on the first response above, "DD" hotels refers to the hotels on Hotel Plaza Boulevard - Disney owns the land but not the hotels. YOUR hotel, Port Orleans Riverside, is most assuredly a Disney-owned hotel and you are eligible for Magical Express. There's NO need for you to PAY for transportation when Disney will get you there FREE.
At any rate, do NOT call Mears for information about Disney's Magical Express. Call Disney.