The most portable solution we've found is to send a Note with InReplyTo containing nothing but a single emoji (which might be a composite emoji) after removing whitespace. The same algorithm on the receiving side also works for Like activities with single-emoji content. We treat these as emoji reactions internally. You can add one or more of the various 'EmojiReact' platform specific cohorts if you want. Just like punycode usernames - it federates with all the things. Unlike all the other solutions we've seen...