I like my scallops kind of "medium" in the center, so for me, an hour at 250 would be overcooked. I agree with searing the scallops, and you just want to get some smoke on them without cooking them too much, so I would try setting your temp to 180 and see if you get good smoke. You are essentially cold-smoking. I don't think you have to wait until the smoker starts producing smoke to put them in. That should happen fairly quickly, before the box temp gets too high. Just make sure you are using chips, or thin slivers that you've shaved off a chunk. Not sure how long to smoke, but 30 minutes or so once the smoker starts producing smoke sounds like a good amount to try. Since you are basically cold-smoking, you could smoke them in advance (a few hours up to a day before your party), then wrap them in plastic wrap and put into the fridge until ready to sear. The rest in the fridge wrapped in plastic would help mellow the smoke, and help it penetrate a bit.