In looking further into this, I strongly feel that my suggestion of "participant of" is a really nice fit for this use case. A few things are needed to make it comfortable for all.
1. Add a new topic in Wikipedia called "Black Lunch Table Artist Roundtable" and subclass it as an Event to Wikidata (we already have "Black Lunch Table")
2. Add the "participant of" statement on each artist or participant at the event.