I finally dug around in the forum settings and found where the minimum number of characters was set. You can now reply with as few as 1 character (so 
and 
are now valid replies!)
You might be able to reply with less than 20 characters, but it seems some type of grammar validation is happening which sometimes doesn’t let a user post with just a single emoji…
2 Likes
Doh!
Try again? I fiddled with the settings some more…
Nope, same result when I try to do a single emoji post
Hmm… must be allowing me because of admin privileges.. The settings are strange “minimum entropy” and stuff like that. I’ll keep trying
^^ oh that worked?! Hmm…
Oh, maybe it’s because I’ve been trying to use emojis outside the built-in Discourse set. Sorry for the distraction!