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Source: BGE Forum, RRP, 2010/10/03
If you can keep the temperature about 120-130F and increase the efficiency of the thermal transfer, the meat will warm up but remain rare.
Best way to accomplish this is probably a kitchen-sink sous vide. Put the prime rib in a zip lock bag, suck all the air out, and put it in a big bowl in the sink with a plate or some other weight to keep it submerged. Get the water running fully hot (usually about 140), then fill the bowl and reduce the flow to a trickle, but still as hot as you can get it. In half an hour you'll have a warm--and still rare--chunk of prime rib.