As PP said, the dining plan must be paid for at the time it is added to the ressie - which is a bit tricky without the renter having to give the member their credit card number.
I recently added the dining plan to a reservation for someone I rented to - she got a Visa debit card for the exact amount of the dining plan. She gave me the number over the phone then I called it in to
DVC. It worked well. If you do this, make sure you give the owner the card#, along with the code on the back and the full name of the person who purchased the card.
Also - hopefully you're asking about next year because the dining plan must be added at least 2 days before the arrival date (I think that's correct).
Can't help you out with recommendations during a busy holiday; we've tended to avoid those times