Not really... that's not much help is it.
Labels as you say, a no no. The "label" side of a disk is only protected by a very thin layer of protective laquer etc, literally microns in thickness. Many pens, chemicals, glues etc can under certain circumstances eat into this over months and years and it can be a major cause of failure.
Maybe the answer is to keep the discs in slimline cases and label those instead, perhaps just identifying each disc with a number written near the center on the "shiny" read side but not impinging on the track data.