I ran a supers game where there was a band of 'subway pirates' who pulled up in their jolly-roger clad underground carriage, leapt out with flintlocks and cutlasses and terrorised a station-full of people before vanishing into the hidden tunnels...
Why not do something similar - an ancient network of dwarven tunnels under the area to be targetted, which the dwarven pirates (perhaps dwarves whose gold seam has run out and who have developed grog) use to emerge from. They then descend in a piratical wave on the town, complete with gold teeth, patches, buckled swashes and earrings, loot what they want and then away.
They navigate the deeper tunnels with pump wagons; maybe a captured purple worm to dig new tunnels to new areas. And that way they can escape from the town in a couple of massive luges - I love the idea of an underground chase sequence in runaway high-speed roller-cars, through a twining network of passages whilst firing spells and ballista bolts at one another.
Maybe, right at the heart of this piratical dwarven empire, a huge cavern with a massive lake in it, and (Goonies style), a massive pirate-ship which the dwarves discovered.
If you prefer the image, the dwarves could ultimately be cursed with 'Black Pearl' style undeath, so they return from the grave with every new moon, filled with vengeance and maggots.
That alternative gives you a ghost-ship of pirate dwarves who simply sail over land.
O'course, if your campaign is set at sea, these ideas aren't a lot of good.
Meanwhile, here's my obligatory pirate-gag.
Pirate 1: "Avast, avast!"
Pirate 2: "Avast what?"
Pirate 1: "I dunno, but it's pretty big."