It's always been sort-of possible to do custom metadata, by either using the Comments field or appropriating a little-used field (like Composer). But yeah, MediaMonkey's ability to label a field is a gamechanger, and then those fields are searchable in any program (albeit, you can't always use custom fields as table headers in other programs)

I use fen0 as a prefix because it's not always clear if you can search including punctuation.