TIL that PostgreSQL has SECURITY LABEL! It seems like this could be useful for storing all sorts of metadata about database objects, not just security stuff. E.g. like the COMMENT but not global. From reading the docs it looks like you need a "label provider" to get it to work though. I can only seem to find a few label providers around, does anyone know of one that isn't security/anonymization related and could be used more generically?

Yep, it's a completely generic feature, and I'd argue the keyword 'SECURITY' is a noise word and 'LABEL' by itself would be even better, but that's what we're stuck with. As the documentation states: " PostgreSQL places no restrictions on whether or how a label provider must interpret security labels; it merely provides a mechanism for storing them."