I'd say give it two hours after activity dies off, then buff up with spells that last for multiple minutes (fly, invisibility, etc.), and hit the airship fast. I say we put a separate spell of invisibility on one person and have them hover above the tower to warn us if the rest of the PCs wake up and come after us. Once we've taken then ship - assuming we aren't under attack - we break the ramp and head into the tower.
Did we end up with anyone that can fly an airship? If the cargo is in it then we could end this thing really fast just by flying away with their stuff.
EDIT: I'm not sure if just having the Least Dragonmark is enough to use the Wheel of Wind and Fire to pilot the ship without a roll (someone who knows Eberron better, is this the case?), but even if not anyone can try to force the elemental to move with an opposed charisma check (most elementals are charisma 10). This means that if the cargo's in the ship or if we get it there, we might just be able to power our way out. Both the cleric and the crusader have good charisma.
EDIT EDIT: Actually, it's probably best to take up the ramp as soon as we get on board, if we get detected, we don't want to make it easy for the PCs to charge up at us.
TRIPLE EDIT: So I looked it up and you need a lesser Dragonmark of Storms to use an airship without a charisma roll, so we'll have to fall back on just hoping for good rolls by the crusader/cleric long enough to get us away. This might be an excellent use of Action Points.