Well, found the problem. The welds broke on clutch shaft, part 11 in the previous diagram. Clutch pedals are not fun to fix or replace unless you want to pull your dash apart. We ended up pulling the shaft, welding a set of splines on it from another clutch pedal assembly, then welding the female end on the original clutch pedal. That way we didn't have to drop the pedal assembly.
It looks like this has happened before, as the welds were pretty ugly and didn't have any penetration. my guess is somebody else tried to weld everything in place, and it just happened to hold for a while.