If you have a layer file (.lyr) saved with the symbology you want, you should be able to have it apply to raster layers within a map document(I haven't tested this).
Since that RGB composite does not fall under a symbology class type that is support by python, you can't change to it directly through the layer's properties(this I did check).
http://desktop.arcgis.com/en/arcmap/10.3/analyze/arcpy-mapping/layer-class.htm
Thanks,
It ran with no problems (got positive gp messages as it progressed ) but, it did not retain the layer file symbology. HOWEVER, I had not considered saving the mosaic dataset to a layer, so loaded the layer instead, and it appears to be a work around .
Thanks again.
In 2022 , ArcGIS Pro 3.0 seems to still have this issue. Hope the improvements will be planned soon for arcpy API.