All I can think is this:
1. Add option "Post Thread After: [ x ] () Minute () Hours to the thread_create template using template modification.
2. If the user has specified a schedule, then I should save the entire thread into my own table (and not XF's default).
3. Run a CRON to simply copy the thread to its original table so that it looks like a regular thread to the visitors.
All I could do is add the options to the template; but that's no brainer. I'm totally confused about how should I proceed with Step #2.
PS: Another option could be to extend the data-writer and set the thread state to 'moderated' and then run a cron every minute and change the state to 'visible' at scheduled time.
PPS: I've already spent several days on this without making any real progress. I'd appreciate some hand holding.