Yeah..the story goes that Brian conned Rieley into doing the lead by getting him to do a guide vocal; several takes later Brian shout's out 'that's it! or something after he gets what he wants. This story comes from Rieley himself and should be treated with dose of salt..on the other hand it does sound like the kind of wheeze Brian would pull.
I have told the exact same story once on the cabinessence board, only to be corrected by Stephen W. Desper who asked where I had heard that nonsense.