Pletscher is still very much in business (www.pletscher.ch) and has a retailer (importer?) at http://www.thorusa.com . If you are lucky, you can find a piece of hardware that sits on top of the brake bridge and receives the bolts that clamp on the rack. They're rubber coated and many probably still knock around in shop drawers. Failing that, make a T-shaped adapter to go over the brake bolt and take the mounting bolts so the rack won't slide down. Wrap handlebar tape to protect the paint from the clamps. I doubt that you will be able to squash the seat stays by overtightening the mounting bolts.
Pletscher makes very nice stronger racks ("Athlete")than the original mousetrap design that have four legs and a variety of front fastening options. Also, Charlie Persons (www.permaco.com) has nice baskets that have hooks on the bottom to engage the Pletscher rack. The mousetrap holds it in place and it can be removed and carried into the market instantly.
I use these racks all the time and think they're great. Nearly every Blackburn-style rack I see has nothing on it and the rider is carrying his stuff in a backpack. That doesn't spell convenience to me!
David Bean
Arlington, MA USA
beandk at are-see-enn dot com