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Question: does anyone know if this would be theoretically moddable, i.e. could one specify a new colour of circuit wire in Lua? Or does this definitely need a dev with source access to do?

I do not think it's possible.

In other hand some kind of multiplexers should be feasible.
You can even create them in base game through combinators.

Internal circuit mechanics can not be modded in any way. Neither wires nor combinators.