I used the /discourse post command from a public channel to great effect… but threads are now more common in many channels than main channel conversations, so still hopeful that some day this will work in a thread…
Update: w00t! just tried it again and it worked!
/discourse post https://ourdomain.slack.com/archives/C0591LA4X/p1529529151000359?thread_ts=1529528304.000177&cid=C0592LA4X
as did:
/discourse post thread https://ourdomain.slack.com/archives/C0592LA4X/p1529528304000177
Another update: I was also able to post a thread from a private channel. 