It's a bit tricky because while you can set the due date on an action, OmniFocus will treat it as having the most restrictive due date of any ancestor, leading to situations like this:
(obviously, this is an action from a project which is due today). If you query OmniFocus with Applescript for a task's due date, it appears you do not get the effective due date (today, in my example).
Maybe the safe thing to do is to at least warn the user that the deferred date isn't actually the effective date? Unfortunately, I think you have to walk up the container chain to do that.