Not that I plan to ever rent, but if I did, what I think I would do is find out upfront if adding the dining plan is something the renters were interested in.
I would then set a time limit for it to be added and make it non-refundable. For example, I might say that it would be added to a reservation 2 weeks in advance and payment in full would be do at that time.
Once added, it would stay so that I didn't have to deal with refunding money to the renter. Hard line, I know, but how I would choose to deal with it.