More parts arrived today (including a friend gifting a well-used Dura-Ace
7800 7900 set, though that's a topic for another thread), namely, the brake booster for the front end.
This thing serves two purposes: 1. It prevents the cantilever 406-to-451 stud adapters from shifting ever so slightly (the centering pins aren't as tight as they could be), and 2. This will serve as the mounting point for the cable hanger that will actually work.
Given how everything is set up, the cable hanger will need to be mounted backwards. Heck, it's not as if anything on this bike is normal in the first place.
Incidentally, the part is a really nice Tektro unit. Can't find any evidence of what they were used on; found this one one eBay in Canada, and only saw one other of its kind on Google Images. It was silver originally; I spraybombed flat black within minutes of it arriving.
-Kurt