I can see the usage case for this even with a "conventional" mixing hot end which might be mixing different filaments (yes it can be done if the temperature requirements are similar). Currently firmware retraction (G10) will retract all extruders contained in the tool definition by the same amount. But if one were mixing a more viscous filament with a less viscous one, then it might be desirable to have different retraction amounts per extruder drive. I don't think retraction based on mixing ratio would be much use (but I could be wrong) because we tend to need the same amount of retraction regardless of filament flow rate. But configurable retraction per extruder drive could be useful. We have configurable pressure advance per extruder drive so why not retraction as well?