I think that would be a nice way to show support and that the beaches are clean/safe - but where could they go that is contained enough that they can actually vacation in peace?
I believe one of the reason's Martha's Vineyard is popular is that it is easily securable, buildings meet Secret Service requirements, and access can be easily controlled.
Somewhere on 30-A might be the best, but can't imagine the cluster a presidential motorcade on 30-A or 98 during the summer would cause. ;-)