That’s a nice idea, but I wonder… At the bare minimum, the tweet will need to contain some forum-specific hashtag and the topic ID. If you have the topic ID, you’ve either just read an email notification from Discourse or have the forum open in your webbrowser.
In either of these cases, going to Twitter, typing in the topic ID and struggling to compose your reply in 140 letters minus the hashtag and topic ID – when you could literally just hit “reply” in your email client or webbrowser – makes not exactly a lot of sense.