I would ask the organizer. Most I've seen have rules that say it must be prepared on site just like the KCBS meats. I've even had the product inspected with my regular KCBS meats. With dessert's, they may only say not store bought. So, you could make it at home and bring it. Like you said, the rules are usually pretty vague and never well defined up front. You may have asked questions up front, but not everyone did. You get there, and everybody understood the rules differently, so you end up with a lot of confusion sometimes. I've seen them say 'no rules' only to find out there are rules when you get there - like turn in containers and trays. That's one reason we stopped doing them.
So, if they say no rules or don't specify, prep in advance. That's how you understood the rules. Just don't tell anybody. It'll only add to the confusion. Good luck.
BTW - KCBS has nothing to do with rules for non-KCBS categories. It's all the organizer.