Restrict work items per timeline?

I am using the Scrum template with RTC 2.0.0.2.
Is it possible to define the following:
- In the backlog iteration, only work items of type Epic must be created
- In a sprint iteration, Epics are not allowed (neither created nor moved to)
I played with preconditions (prevent editing), permissions (create work item of a specific type) but nothing did the trick. After restricting permissions, I could not create any work item but an Epic in any plan. After restricting the operation behavior for Save Work Item, I could not edit an Epic in any plan any more although I override the operation behavior for one sprint only.
What am I doing wrong?
Any help?

Thanks
- Thomas