Proof of concept - I had some brief tests where it didn’t work which I think are related to whether it’s fast enough to modify the data, but usually it seemed to work. I doubt this approach would work with fetching another site’s emojis in real time for that reason, but if a URL can get plugged in somewhere and the result cached into local storage or Indexed DB it might be fast enough to read the data from one of them.

// ==UserScript==
// @name         Lemmoji
// @namespace    http://something.here/
// @version      2026-07-13
// @description  add emojis to lemmy
// @author       Trying2KnowMyself, theoretically with some code stolen from Edie except that I've only made it to a proof of concept phase so far and didn't actually steal code yet
// @match        https://hexbear.net/*
// @icon         https://hexbear.net/pictrs/image/79f0e1d8-b425-47fe-a1ac-9ee1ab6c246d.png
// @grant        none
// @run-at document-start
// ==/UserScript==

(function() {
  'use strict';
  // it might already be set
  let isoData = window.isoData;
  if (window.isoData) {
    console.log("isoData already defined, might not work?");
    augmentCustomEmojis(isoData);
  }
  Object.defineProperty(window, 'isoData', {
    get () {
        console.log("getting isoData");
        return isoData;
    },
    set(value) {
      console.log("setting isoData");
      augmentCustomEmojis(value);
    }
  });
  function augmentCustomEmojis(value) {
      console.log("augmenting isoData");
      const biggestId = value.site_res.custom_emojis.reduce((acc, curr) => Math.max(acc, curr.custom_emoji.id), 0);
      value.site_res.custom_emojis.push({
        "custom_emoji": {
          "id": biggestId + 1,
          "local_site_id": 1,
          "shortcode": "catclops",
          "image_url": "https://lemmy.ml/pictrs/image/fbab8b2d-2189-4311-b26f-61ae7cfa632c.png",
          "alt_text": "catclops",
          "category": "Custom",
          "published": "2023-06-17T21:15:35.303290Z"
        },
        "keywords": [{
          "custom_emoji_id": biggestId + 1,
          "keyword": "catclops"
        }]
      });
      isoData = value
  }
})();

It was interesting to see that /api/v3/site only gets hit on login - I briefly locked myself out with a login rate limit. Even more interesting to see why: isoData gets set via the HTML if you’re already logged in.

Some general thoughts after poking around at it a bit + prior userscripts experience:

• userscripts modify the page, so displaying it would mean inserting it somewhere on the site rather than having an extension button to show it - that leaves two possible routes: replace the emoji picker on input boxes, or throw it somewhere common like the site header
• if options are not hard coded, they would similarly need to be rendered somewhere to support config changes - hard coded would be an easier starting point to get the rest of it working
• throwing it somewhere common is probably easier, because the general approach I’ve seen/used previously is that you create a promise that loops trying to select a known element with a setTimeout some number of times, and appends/prepends the desired element to the container it finds
• getting it to replace the built in picker would probably require first using that approach to attach event listeners to reply buttons and then using the events to trigger the replacement behaviour when someone starts to comment + additional handling for the create post page
• best practice for userscripts is to restrict the URLs they run on - something that potentially runs on your bank website is much more concerning than something that only runs on one site, but it’d be difficult to limit it appropriately enough while still listing/supporting most instances
• some userscripts will also intercept network calls by overriding methods on XMLHttpRequest.prototype and attaching load event listeners when the path matches, though I’ve mostly seen/used this in the context of wanting to capture the results to do other things with them - if that or similar could instead be used to augment the emojis response, then that could allow additional data to be added to the existing picker without replacing it

I’ll probably take another look at things later, but figured I’d share my thoughts so far.