I had a similar situation trying to declare submenu items in a RibbonDropDownButton control as regions.
For me this thread helped:
I've modified the RegionManagerRegistrationBehavior::FindRegionManager method in Microsoft.Practices.Composite.Presentation according to the above sited post:
... (line 161)
parent = LogicalTreeHelper.GetParent(dependencyObject);
if (parent == null)
parent = System.Windows.Media.VisualTreeHelper.GetParent(dependencyObject);
if (parent != null)
Hope it works for you.