I was just changing my Main this weekend and was looking at the tack pin arrangement on Knotius Maximus. Similar to Steve, my Main uses slugs so we don't take it off every day and hence a fast pin wouldn't be of any advantage. My tack pin is located just below the reefing hooks which is bolted on to the boom - mine is a 5/8 diameter (approx) clevis pin with a split ring on one side an a cotter pin on the other. I can see that if your main has a Bolt-Rope style luff then it might make sense to take the main off the boom and a fast pin would help. Long story short - I don't know if there is a mkV 'standard' for the tack pin - I do know that my mast is different that Steve and Jim's.
Hope this helps.
Mk V - Hull 154