I was with you until you said stop at MCO. That would make the bus ride from Sanford the third gate of hell.
The only way I can see a smaller airport serviced by DME would be if the airport was closer to WDW than MCO. Not the case here...a quick Google maps check has SFB to Caribbean Beach resort (picked that at random) at 55 minutes. As I said earlier and mentioned by others, the only DME service that would even be feasible would be a SFB/MCO shuttle. Allegiant just doesn't have enough flights to justify a DME fleet and the extra distance makes it even more difficult.
I agree that it would be less than ideal to do SFB -> MCO -> DW on DME but I think it's better than nothing. If DW is hesitant to offer DME from SFB, I think this is a baby step to get them to try offering it. If there's enough demand, I'm sure they'd switch to SFB -> DW direct.