The due date calendar is provided as a means of getting reliable notifications of impending due dates on mobile devices, not as a planning tool, and while it can be put to some use in the latter capacity, it will be a frustrating experience for many. You get no idea of the duration of something, only the point at which it must be completed. The due date reminders calendar only has 0 minute appointments, to trigger the local calendar alerts. Knowing that you have 6 things due Friday at 5 PM won't help you plan your schedule if you can't discern whether they are tasks requiring small amounts of work (file expense report) or large (prepare department budget request), though it might help you decide whether or not you can be out of the office that day (oh yeah, have to file all of that paperwork, can't telecommute/play hooky).
But let's say that we had a script that would build up a calendar showing your due dates for various actions, and would use the duration field to make a starting point for the action the requisite number of hours in advance. Now you look at your calendar for Friday and you see 6 overlapping blobs, all ending at 5 PM. That still doesn't seem like much help, unless you are going to adjust all of the due dates until nothing overlaps. Why not just look at the actions due this week and block off sufficient time in the calendar to work on each one?
Have a look in the OmniFocus iCal sync forum for a script that does what you ask. You aren't the first to ask for such things nor will you be the last. This might be a good starting point:
http://forums.omnigroup.com/showthread.php?t=11331