[rmrector]

But shouldn't extrafanart just be provided by the core art loader without the skin needing to know where it comes from? Am I missing some other issue here?

That's definitely the way I see it.

Mostly extrafanart for music is relevant for the now-playing song.  In some cases it is useful for the focused item in lists or in the music or song info dialogs.  I don't have a need for displaying extrafanart within containers and don't really see a utility for that.  So the need is just for musicplayer or the focused container/listitem.

Kodi's internal artwork support is meant to be in the library, an add-on can implement other behavior. The better way to go here is to expose the artist folder for JSON-RPC to facilitate that than skins cobbling together paths, as Dave mentioned. FWIW my video library art tables has 116k artwork.