I'm only an FO, so I don't get upgrades very often, but I do find that when I don't get an upgrade the gate attendants usually stick me in a bulkhead/exit-row seat - sort of a sop to those of us who didn't qualify for first/business class. It might be that you simply need to fly Delta a bit and ask. I don't know what the rules are vis-a-vis disabled passengers, but I assume they have special, separate accommodations for such flyers.