I guess I don't see how this is difficult? We do it all the time. A lot of times, if we go out with friends and don't ask for seperate checks, someone doesn't have cash. At that point, they usually offer to just put it on their card and the other parties hand them cash. If two people don't have cash, someone offers to put it on their card and the other party either 1) picks up the tab later or 2) hits an ATM after the meal. It's not hard. The prices are all available at the time of pre-ordering, I'm pretty sure, so everyone can see their total as they order and even if you don't, there are plenty of places to find the prices for dividing later.
In my mind, Bob and Judy are dining with Joe and Carol. Bob and Judy have the dining plan and Joe and Carol don't. They order and Joe and Carol pay for the lunch. Bob and Judy pick up lunch later and that pays back Joe and Carol. That seems easy, to me.