I've just finished and uploaded the scheduled posting code. It is now possible to schedule posts for publishing at an arbitrary future date.
I've taken advantage of the save-for-later code, so that a post which is scheduled to be published is merely saved for later with a schedule for its posting made. This way one just uses the regular ability to edit saved posts to edit scheduled posts (plus canceling a post just means deleting it from the saved posts).
I also have to give credit to Date::Manip for allowing me to accept dates in arbitrary formats like "Next thursday at 5am". For anyone who's a perl programmer, Date::Manip is a fabulous module if you need to take a date from a user (as the author himself says, though, it's far too slow for repetative parsing, such as in a tight loop).