If I have this right, there is some kind of brace between the stem and fork, possibly to keep a long stem from flexing in a sprint. Well, you are not going to want to use that stem and bars on the road, anyway. It's too much hassle to mount and unmount the brake lever and recover the bars. When you swap the fork, swap out the handlebar & stem too. You'll save time and have your brake. A front brake is way better than a rear brake on a fixed gear, and you're likely to leave marks or dents on the frame with anything you clamp on--especially considering the forces involved.
If you do decide to clamp something in the rear, you will need to go out to the seatstays. I can't imagine clamping anything to the brakebridge that wouldn't slip under braking stress, or damage the bridge. Schwinn made clamp-on steel cantilevers, but that's going back a loooong way.
Steve Barner, whose wife won't let him ride the track bike on the road anymore in Bolton, Vermont (the roads are all dirt, anyway--or is that sour grapes?)
> Date: Fri, 25 Apr 2003 12:55:00 -0400
\r?\n> From: Martin Needleman <mrndlmn@toad.net>
\r?\n>
\r?\n> Sorry folks. I didn't give you complete information. The "swap the
\r?\n> forks" option is not available. The front fork of the bike is actually
\r?\n> drilled half-way through to accommodate a brace that runs between the
\r?\n> stem and the fork. My limited defense is that I'm not entirely sure that
\r?\n> this bike is technically on topic. It was built sometime in the eighties
\r?\n> by Pino Morroni and I was trying to avoid discussion of the [possibly
\r?\n> out of age range] bike itself.
\r?\n>
\r?\n> Martin Needleman
\r?\n> Annapolis, MD