When the Grease Pencil Merge happened, I had this idea of using such GPO (grease pencil objects) as custom shapes for the bones in my rig. Instead of painstakingly modelling some weirdo gizmo shape, one could simply sketch it. As a benefit, it would also not add a lot of weight to the file itself.
Current status is, I can set the GPO as a custom shape, but it is not displayed at all unfortunately. See the two screenshots below, where the bone simply disappears:
