While I don’t agree that we’re ‘paying’ for
Free Dining via rack rate for rooms (check out the rack rate on a non-Disney hotel some time; the information may well be on the emergency notice on the inside of your room door. I stayed somewhere in Las Vegas recently, and while I paid between $0 and $35 per night, the rack rate for something equivalent to a Moderate is $400, but I digress), a family of four is effectively getting a discount of $127.96 per night, plus DISNEY – not the Guest – pays the gratuity on all meals for which DDP credits are used as payment.
The explanations and suggestions offered by the other posters in this thread make absolute sense. In my opinion, it appears that what the OP intends to do is pretty much the same as the relatively insulting (to the server) and ideally rare practice of placing a stack of coins or bills on the table and telling the server at the beginning of the meal that this will be their tip - and then removing money bit by bit if the customer isn't happy.
In all honesty, if one goes into a situation expecting the worst, one will generally find the negative in that situation.
In direct response to the OP's question,
What do I tell them when they ask if on the ddp?
you could try telling them exactly what you intend to do, and see what response you get. You might be right - it might work.