I think a brokered convention is quite possible. For the Pubbies to haul out another Bush—all the more without his having campaigned!—would signal both idiocy and all the more proof that the Bush family’s influence on the party must be deflated (just as the Dems are daring to do with the Clinton’s now).
What’s important I think is the Thompson has low negatives, and with the other guys doing the dirty fighting they should stay low up until the convention. And, if he can bring a reasonable # of delegates to the party himself, it will look as if he has earned it if he becomes the consensus candidate.
The problem with your theory is that in a brokered convention superdelegates have a lot of power. These superdelegates tend to be RINOs. I could see FL, TX, CA, MN, MD, etc all go for McCain instead of Fred. I am a Fred supporter but I would not bet on him in a brokered convention.