PieFed v1.4 (to be released very soon) will have emoji reactions on posts and comments. An emoji reaction can be a unicode character like 😍 or a colon-delineated string like :piefed: which gets automatically converted into an image when the post/comment is displayed.
Setting a reaction uses the same API endpoint as casting a vote (and behind the scenes reactions are federated as Likes, for Lemmy compatibility) but with an emoji element added to the JSON payload.
Also when looking at a post or comment in the API there is an emoji_reactions element that can be used to get a summary of the reactions that have happened so far. Compare https://crust.piefed.social/api/alpha/comment?id=456791 with https://crust.piefed.social/post/84566#comment_456791 to see what I mean.
Do a GET on /api/v3/site to retrieve a list of all the custom emoji (the ones with :something:) that this instance knows about and to find what images to convert usages of them in emoji_reactions to.
Interesting. It shows up, but just as the shortcode:
@rimu@piefed.social - not sure what the json looks like coming from mitra, but at least in the short term it might make sense to just have piefed interpret emoji it doesn't understand as an upvote (like it had been doing up to now) so that it doesn't break the UI like this.
Also adding :catjam: might be a good idea—it seems to be the ruling emoji of the fediverse.
Edit: I might have just missed it on account of the disco theme.