PiP '07 was my first full blown pirate event and I heard scuttlebutt in the campsite about being on a cannon crew. Since I was writing the surgeon's journal, I decided to do anything I could related to the event so I would have something to talk about. I asked Mercury Quartermaster William Red Wake how to get on one of the crews. He told me to go to the fort about an hour before the battle began (2pm) and hang around where the cannon were. Like you, I was looking for some sort of commitment, so I asked someone there (probably Harry) about firing a cannon. As a result, I was paired up with Mark - a local volunteer - who taught me how to fire a cannon in the field. You can read my (brief) account of that here.
Since there are cannon in the field and in the fort, there are several opportunities to get on a crew. As vintagesailor said, the Viceroy has several on the fort wall. They fire them during the battle and at sunset. There are three battles and three sunsets resulting in lots of opportunities. You do have to be there early enough to get trained on your role in the crew (which varies - depending on how many people there are volunteering.) M.A. d'Dogge was in charge of the crew I was on last year - we had a team of five or six people (you can see my account of my '08 experience here.)
It really is mostly just a matter of being around about an hour or so before battle from my experience. They're nearly always looking for volunteers.
Alright cool. Thanks again for the help Mission.