I worried about that a little at first but didn’t experience the housings moving at all. The only thing I see is the free sections of the cable going slightly slack, hardly enough to notice unless you pluck them.

I used an inline barrel adjuster on both cables and that works great to fine tune them without messing around with the pinch bolt.

How about find an late 80's MTB frame from when they mounted U / roller cam brakes under the chain stays. Then braze some canti mounts on to the seat stays. One lever for each.

Alternatively, you could get U brake mounts brazed on to the chainstays on a canti bike. But you'd have to make sure that specific frame could handle it.

Yeah Im not going to get into brazing, just because I do not think this frame is worth repainting. I will probally build a new rig over the winter. So I will stick with a shot reach brake mounted normally and then a longer reach brake with a Metal plate mounted behind that.

You could drill into the side of that alloy part and then makes some threads and put one of those little hex screws through it (like the adjusters found on bmx style levers) to hold the cable housing there.
Also there was another flaw in my gyro cable system. On the metal bars the parts holding the bottom barrel adjusters they where attached to my stem but they would slip no matter how much I tightened them. I know there is a way I could get around this but It was getting to be to much..

BMX market has adapter plates to go from long reach side pulls to canti/v brake

such as https://billys.co.uk/english/group.php?prod=bsdnvps

or to U brakes.. a U shaped piece with 2 brake bosses.

You could get 2, bolt them on both sides if the seat stay bridge,

then get 2 of the same brake to mount onto them..

1 lever 1 brake , but since they are both on the same wheel, task accomplished.