Does this work?
It does work! So you can now reference albums and tracks from https://open.audio/library in Mastodon via their url and get an embedded player for free!
(it will land in Funkwhale 0.19, but it's already on the develop branch if you want to try it ;)
My initial mistake was to believe that Mastodon was relying on OEmbed to do this.
In fact it uses twitter:player:* Meta tags x)
This probably brings compatibility with other software/websites out there that support those meta tags, but I don't know which ones :D
mastodon.eliotberriot.com is one server in the network