I did a 10 lbs. boneless rib roast for Christmas. Three hours twenty minutes on the WSM at 225F brought it to 118F internal. I pulled it, covered loosely with foil, covered that with a kitchen towel, and let it sit on top of the stove. Over the next hour the internal rose to 129F. Due to some miscalculations on the timing, it sat there for a couple hours more, eventually cooling to about 110F internal before everything else was ready. I took the middle and top off the WSM and let the still burning coals get nice and hot. I set the grate on top of the charcoal ring and then put the roast back on for a couple minutes per side. The last side didn't get quite the desired time because the fire was too hot and too much grease rendered from the first side, resulting in too much flame. Still it got nicely browned. The inside was uniformly reddish except for maybe a quarter inch around the exterior.
This is the second time I've used the reverse sear technique on a rib roast and I now can't imagine doing it any other way. It's almost foolproof since the slow heat gently brings it up to temp. The rest allows carry-over to bring it to the proper internal temp and stabilize so the final sear doesn't drive the internal temp too high. Since it has already rested you can slice and serve immediately after searing. You could, of course, sear it on the grill or even on the stove top if you have a pan large enough.
The only thing I didn't like was the flame-up when I was searing it. I left the lid off because I wasn't expecting to leave it on for more than a couple minutes. I think it probably would have been better to leave the lid on to tame the flame. Even with the flames the results were excellent.