Hello Sébastien,
You can pass local coordinate system as default value:
crs = Metashape.CoordinateSystem('LOCAL_CS["Local Coordinates (m)",LOCAL_DATUM["Local Datum",0],UNIT["metre",1,AUTHORITY["EPSG","9001"]]]')
Metashape.app.getCoordinateSystem(crs)
But please describe a use-case, when it is required to ask for the local coordinate system selection in the georeferenced project?