Yes, we did this last year several times.
At Boma, we ordered a pressed pot of coffee which wasn't covered under the dining plan. We charged the coffee to our room, and our 4 meals were covered under our dining plan.
At Flame Tree, we ordered an additional side of french fries which we charged to our room.
We also had planned more table service meals than that for which we had credits. So, the least expensive ones (e.g., Trails End buffet) we charged to our room card, and this was before we had used up all our credits. We just told our server that we wanted the meals charged to our room & not from our dining plan credits. When she/he brought our receipt to us, we just made sure that it was done that way, and it always was.
(I did keep track of how many credits we had, how many we'd used, & how many we had left so that when I looked at the receipt I could tell if we'd been charged correctly.)
No problems!
On the 1st couple of days, I brought cash w/ me in the park, but, by the end, all I carried was my "Key to the World" card! It was so fun!