You want a rear brake because.......you dont have a coaster brake?
For the work it will take to put on a caliper, and the (to me) marginal performance of them, I would be looking at a canti or V brake set up and end up with real brakes. I really like the coasters though, and the clean look/no cables/no messing around you get with them.
I agree 100%. Have a set of mtn bike wheels that I was going to use. Figured it would be cheaper than having the rim relaced with the coaster brake I have. It's gonna be a straight rat rod bike.
My neighbor the professor wanted 6 speeds on the 64 Starlet I built for her, so I had to make up a bracket to mount a rear brake to. The frame already had holes drilled in it from a previous modification, mine was heavier to to take up the strain of the brakes. Otherwise I wouldn't have drilled holes in the stays, I would have used two plates bolted on either side of the stays.
The two plate would stay in place if one has a couple of stops welded on the underside (one to the top and one to the bottom of the crossbrace)to help hold it once bolted together.