At a Morris Ale in 1990, I watched a really bad pirate skit done by the Hearrrrrts of Oak. It was so bad that it was great and left everybody for the rest of the evening and the last day of the Ale continuing to add joke lines. Since you are doing a concert, a skit-like thing might work well. Use 4 to 8 people who like to ham it up extemporaneously. Essentially no rehearsal required, but some simple pirate suggestive bandana's or other pirate paraphanalia, especially hand props are useful. Here is the routine:

The pirates are all up on stages slowly meandering or standing saying "Arrrr", "Arrrr, Arrrr", "Arrrr", ad infinitum. Finally one of the slowly gets the others attention much to their disgust and asks, "What kind of stockings do Pirates wear when they storm a ship? The others all keep saying arrr in a questioning manner and perhaps their are some bad answers such as "Nylons" to which they all start snarling, arrr in disgust with the inference. Finally someone offers the correct answer "Arrrrrgyle" to which all the others arrr in approval or more disgust at the bad joke. This can go on with such things as arrrrrrtichokes, arrrrrrrthritis, or anything else that is inspired by the evening. If they are singers, you might climax it with having them sing some old pseudo pirate song in "Arrrrrrrrrrmony." This is also one for arrrrrrrrrdience involvement who can easily relate to the blatantly simple routine. The whole thing carries itself with little prepartion. Planning the climax and ending is the only thing that needs some good thought. Arrrrrrrrdapt to your setting, it's very varrrrrrrrsitile.