I think this would be fine. I think the success will be in the business plan. With something like this, you could try out various strategies, paying attention to what people on this board say and elsewhere. You could try it out without too much to lose. A shuttle back and forth to the airport(s) with a competitive rate would be good. It costs a minimum one way of $65 for a taxi from the Panama City airport to get to Rosemary Beach area. Keep in mind that if a family of 4 is travelling together, taking the shuttle could become expensive so there could be family plans possibly. You'd want to figure out the different benefits/costs of people renting cars (expensive but a lot of flexibility and may not be too much more expensive than buying bus tickets for a family of 4/5 every day), taking taxis if a family only wants to leave their area 1-2 times each week, versus the flexibility of a van option like yours. I do like the idea of a pick-up and drop off at people's communities if the van parks off the street and doesn't get in anyone's way. Making the van more beach like would make it more lovable, though I might not paint it up until I tested out various options, did the advertising, figured out permits, etc.
So far, we've always rented cars for our family of 2-4 people because of the flexbiility, but in general we just drive from the airport to the cottage, go to a few restaurants and grocery shopping during the week, and then drive back to the airport. So, vans may be able to offer a better rate. We'd love to come to the beach without renting a car. And I agree that taxis and shuttles are a great service to keep people who are drinking off the road after visiting bars/restaurants.