Since the rear brake works, I was assuming a clean up of the pedal was all that was required to tidy up this part of the bike. A load of gunk noticed whilst the battery was out. I assumed incorrectly!
1 Pedal shaft bearing tube seized to bushings. The pedal was somehow moving on the (properly tight) central bolt. This took a drift and favourite hammer (again!) but after some cleanup had little play and caused no issue.
2 Rear adjuster barrel (the part the brass nut touches) did not rotate in the arm coming out of the drum. This leading to the next issue, since I could not rotate it:
3 Arm coming out of drum seized onto spline.
The spline took about a half hour of carful levering, prying open the split and spraying. Eventually coming off with a puff of white oxidisation. The barrel would not shift and I had to get the blowlamp to it. Being made of BMW metal, everything cleaned up and did not get mauled in the various shadetree blacksmithing processes.
That little clip on the pedal arm of the linkage rod is a pain. I did it without removing the exhaust, rear wheel or fingernails, but don't plan on taking it out again soon. So yes, probably worth crawling over absolutely every inch of a 40 year old bike, not just the obviously faulty parts!