Dawes-man, I realize in your last post you have already (and rightly) rejected this type of option, but another brake that would be period correct and doesn't require drilling the frame is the Philco Centralpull such as this one that I had installed on my Fothergill for a while:



It clamps to the stays. It would be possible to put it under the chain stays. It's not a very strong brake (perhaps the fault of the levers I used it with), but it would probably satisfy the legal requirement.