I've been poking around SubText's SQL area and it can be done but not very effectively since how things get updated.  I can modify subtext_GetConditionalEntries with "AND DateAdded < getdate()" but when you update the post and want to modify the viewable date, this isn't enough.  So I thought the DateUpdated would work and it would work for the 1st time you post but afterwards, it updates with the current time, not the wanted to post time.

SubText, in my opinion, as it currently stands cannot support future posting.  I'm not willing to modify the engine this much to add in this ability for fear of breaking too much stuff.  Shame since this is one of my favorite features in Community Server.