I don't see why you can't move the tasks around as the day fills. Start dates aren't write-once values. I do think that planning out your day like this is perhaps wasted effort when carried beyond the minimum. Pick out the items you must do today (your big rocks). Work those to completion. Now work on what is left (little rocks). If you need an uninterrupted block of time to accomplish a particular rock, put an appointment on your calendar for that block of time. It will show up in the Forecast view on iOS, and the Forecast view is a good way to drive the work. Look at the calendar section, and see if you have an appointment. If you do, go to the appointment, and restart process when you return. If not, look at the due today portion of the display, pick one of the actions due earliest in the day, and work it until done (or the next calendar entry starts). Restart process. If you run out of tasks in the due today section, or you can't work on any of the remaining ones just yet, look at your flagged list. Take one from there, work it, restart. If that is empty, the started today portion of the Forecast view would be a good place to look (getting the tasks out of the system as quickly as possible). You'll restock the flagged list when doing your reviews.
If you're set on putting everything in the calendar, check the OmniFocus Extras and iCal Sync forums for scripts that turn actions into calendar entries. In particular, I believe Brian has contributed such a script here:
http://forums.omnigroup.com/showthread.php?t=11331
RobTrew has the framework for a different approach here:
http://forums.omnigroup.com/showthread.php?t=23927
wilsonng has some good points in page 3 of this thread:
http://forums.omnigroup.com/showthre...t=18570&page=3
Bottom line is that OmniFocus is built to support a style of working that doesn't rely on planning out the time at which you'll work on every task, and you may encounter some friction trying to use it in a more planned out fashion.