I did a full 7lb sirloin in a DO once on a Deso trip, dried it really well, coated with sugar, put directly on the fire to caramelize the sugar and create a good sear / crust, then into the DO, 6 coals on the bottom, 4 on the top, about 40 minutes later checked with a meat thermometer, was perfectly rare / medium rare depending on where you cut. And yes, if memory serves we replaced the coals when they burned down with coals from the fire.
I don't see any reason you couldn't do this with a small prime rib. The sear is key in my opinion, and instead of regular white sugar, I'd use Brown sugar, rosemary, a little sage, smoked paprika, white pepper and a little cumin on a prime, but that's just me
