We've made some progress. We haven't had a time out longer than five minutes in the past few days. It took some time to train him, but Peter seems to get that the sooner he gets quiet, the sooner he'll get out.
He also seems to get that I will put him in time out anytime, anywhere, including Cracker Barrel.
And putting him in time out for his tantrums has been absolutely liberating! I've never know quite what to do with those tantrums, and my past attempts have always failed, from the cave-man empathy a la Karp to spanking a la Dobson. Sending him to his room always made me feel a little guilty, but it never seemed right to have him blowing his top in the living room either.
After a week of boot camp, though, when I put him in time out, he quickly settles down. (I should add that the time out is in a patch of the living room, so he's not isolated from the rest of the family.)
I've long known that time out was originally created as a way to help an overstimulated child settle down. Whether you see it this way, or as a punishment, the result for us has been the same: Peter does settle down. It's like pulling the stinger out of the wound.