You could do as I described in this post:
Organize tasks inside ”Today” Forecast
If you don’t have the Pro version of Omnifocus, you could read the post after that in the same thread.
Personally I have made my own Forecast perspective, including a list of tasks with a special Forecast tag. That list can be manually re-ordered, and when I drag tasks to that list they will get the tag automatically. That works fine for me.
