Interested in this feature as well. My use-case is a bit different.
We use Discourse for work and as the team lead, I want to encourage people to have a healthy work life balance. However, there are times when I am ready to draft a reply for a topic on weekends/out of office hours, so I want to write it to get it out of the way, but schedule it to only get posted once work hours resume so people don’t get pressured to respond outside of office hours.