It's not too hard to slip a new wire into a laced bundle. In the places that still use lacing (aerospace), they use tefzel insulated wires that have the benefit of being super slippery against each other. This makes it easy to slip one in but it also helps reduce stiffness in the bundle because the insulation isn't sticking to itself as much as other things like vinyl.

Last cable I made: https://ibb.co/4gw3GQGL