The reason they wont take a rear brake is they are true track bikes. If you want to get a really nice frame like a cinelli you need to understand that at that price range most all of the bikes will not accept rear brakes as they were not made to do so. They are also at that price range almost exclusively made for the track. Im not saying you cant use one on the street because alot of people do, but its going to suck when you dent it and even worse when you wreck it.
But really, why do you want a rear brake? The front brake provides id say 70% of your stopping power.