I hope that Omnigroup adds start dates into their forecast view as well, altho it already exists in iPad version so I suppose they will implement it into iPhone as well.
An alternative solution for both of your requests (reminder before Due Date / Start Dates in a forecast view) is that if you start to use a method where you use 'Today' perspective. You can define a task to appear in a Today perspective before the actual Due Date by defining a Start Date.
Then you can create a 'Scheduled' perspective where you can see your upcoming tasks based on their start dates just as you would in forecast view with start dates.
To create the perspectives you will need the desktop version of Omnifocus.
It does take some effort but there is a detailed description how to make all this here: http://forums.omnigroup.com/showthre...=things&page=2