Manston should investigate the promotion of fly/cruise packages.
This is one area in which they definately have the capability to accomodate the largest aircraft and the catchment area to make it commercially viable.
I would suggest that it costs operators a fortune to put such charters on from Gatwick, and a reputable agent, such as Newmarket Travel etc. have the contacts to advise accordingly.
Even if it's only 5 or 6 flights a year, that's 3000 to 4000 passengers using the airport which weren't there before.
If you are thinking of connecting flights to Carribean cruises, these often use Gatwick based aircraft from the likes of TOM, so the positioning flight to Kent/Manston would probably negate any cost savings