Hmm no sry there is nothing that helped me out.
This “material override” is a field at the detail list of the object, you can add meterials in it but there is no way I can get it in a blueprint.
My thoughts are that I try to fade from the original material to the material override and back. This must be handled in a blueprint and there is no chance to use the material override.
If someone knows a better or other way to fade between 2 materials then this would be good too.