Is there a standard reference designator to use for labelling indicator lamps in schematics?
In fact there is a standard: IEEE Std315-1975 Graphic Symbols for Electrical and
Electronics Diagrams (Including Reference Designation Letters)

The standard specifies a reference designator of "DS" for a "general light source", "lamp", and "signal light". It has the same reference designation for "light-emitting solid-state device", by the way.