Lovely. I'll have a play around with it.
I did try :-
"infill_extruder_nr": { "label": "Infill Extruder", "description": "The extruder train used for printing infill. This is used in multi-extrusion.", "type": "optional_extruder", "default_value": "-1", "settable_per_mesh": false, "settable_per_extruder": true, "settable_per_meshgroup": true, "settable_globally": false, "enabled