This is a teeny weeny bug, probably related to :discourse2: being bespoke or something. It would be nice to have it in the default emoji sets though in other instances. We like and refer to the mothership quite often!
I discovered this when Oneboxing.
It renders nicely here in meta:
But not in mine:
FWIW, I find this idea appealing. 
I just tried a workaround of adding it as an image instead, but that got pulled out as the onebox thumbnail so didn’t really work as well as I’d hoped.
We only ship official Unicode emojis in Discourse, and leave for admins to add their custom emojis easily in the web interface.
No space to slip a cheeky bit of branding in? 
Maybe a special little something for the admin guides, etc?
It has taken me years to finally stumble across how to do this!
The settings are in /admin/customize/emojis, and the documentation is here:
This isn’t surfaced very easily in a search for emoji (for example, add emoji). You can only change the search priority by category, not by tag eh?
Also, I wonder if this old emoji blog post should be updated:
