A 4mm ferrule does fit over the metal pipe of a brake noodle. I replaced the junction ferrule with the 4mm ferrule and placed a Jagwire in-line adjuster over that. I shortened the housings leading into the adjuster from the handlebar and hooked the cables back up. Could it be that easy?
Millimeters matter. The shifter noodles did not swing smoothly with the adjusters on them. The front brake came up against the shift cable going into the stop on the left side of the head tube. The rear shifting was still unreliable.
After a few hours in a cold garage, trying different possibilities I almost gave up and stuck it together with doomed housing going straight into the head tube stops. But I hate to let go of a good idea, and shifter noodles are a good idea on bikes with these infuriating defects on the head tube.
I tried using a short section of flexible brake housing on the front shifter cable, since it doesn't require perfect indexing. The springy shift housing pulled the flexible housing out at an awkward angle. The part that leads into the cable stop has to be rigid to feed the cable around the tight curve at the head tube.
In the end I had to grind off a couple of millimeters from the cable stop on the left side to make just enough space for the bars to swing unimpeded. The computer wire had to be led straight up the front of the brake cable to preserve the clearance.
Here's a crude video overview:
The real cure on any of the afflicted bikes would be to saw off the stupid head tube cable stops and put on a set of stops where they will actually work. But the industry wants to move into electrical shifting anyway. Batteries not included, of course.