Thanks, that makes sense. Sort of like chroma-key in video (in my youth I designed video equipment).

Without a special tool for it, I think it would be tricky, but gimp may be able to do it in a reasonable way. The first thing I would do is to look at the range of colour values in the line, and take it from there.

Maybe this would be a worthwhile challenge to offer people? You can't be the only person needing this.

Meanwhile, if by some remote chance you are near the same bit of planet that I am, I could repair/replace the dodgy cables so that they stay repaired. I live in Bedfordshire, UK.