poVoq heh, you and I seem to be on the same wavelength because I'm interested in this feature too.
According to this, Flarum has changed from storing shortcodes to unicode for emoji. That's gonna seriously complicate any implementation of custom emoji.
it would seem that shortcodes are still presented to the user, so we might not be completely out of luck. it'd take some tinkering to figure out how to handle this and how much work it will be. (I've got a full plate atm and really need to finish some of my extension projects before I begin any more)