You still can not use a turkey fryer at all, even to boil water. If that water is being used for the hot dogs then the answer is still no.
If you were using it to boil water for coffee then yes. I know it is confusing and really makes little sense but that is the way the rules are written. Like I said in my earlier post you can only use the cart for cooking anything to do with a hot dog.
One thing to do to know for sure is to contact your local health dept and ask them yourself. You will have to notify them that you are doing the event anyway because that is part of the rules of a cart also. Ask them about doing extra dogs in a fryer. Do you have a back up plan if it rains? Tent?