Elytra supports WebSub as well as RPC style pings. So I made a 3-step Shortcut that fetches data from a xml file and then calls the Elytra API URL to handle the RPC ping. This file is preloaded with information about my microblog.
You can download the shortcut here. Remember to change the contents of the xml file if you plan to use it. You can adapt this to send RPC pings to other services too.
Now I just need to figure out how I can automate this process when using Icro or the Micro.blog iOS app after I submit a new post. If you have tips on that, please let me know.