I agree that it's a good idea to call Disney's Magical Express (DME) Guest Services at 866-599-0951 to confirm the plan. It's true that Magical Express does not allow "luggage-only" transfers according to the
Magical Express FAQ. The OP's plan is to have at least one person in the party checking in and riding a Magical Express motorcoach not expecting a "luggage-only" transfer.
There are two luggage tags per person in the DME packet, so a party of 4 will have 8 luggage tags. Let's suppose that the party uses all eight tags, but only one person checks in and rides the motorcoach. So the question is whether all eight bags would be transported and delivered, or only the two bags tagged with the name of the person who checked in.
See post 25 in the thread
"ME bus or towncar?" in which CleveRocks discussed exactly that question with DME Guest Services. The answer was that all 8 bags would be delivered, because the delivery is tied to the reservation number, not to the name on the tag.
Again, I would recommend that anyone who wants to do this should double-check with DME Guest Services first.