I am not sure I fully understood your question, but perhaps you can use
VisualTreeHelper
?
Link:
http://msdn.microsoft.com/en-us/library/system.windows.media.visualtreehelper.aspx[
^]
On the other hand, if these controls implement the pseudo-inner controls purely via custom rendering/painting (without using any elements at all), then you may be out of luck.
[Update based on our discussion via comments]
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Alright, I understand your problem better. My previous assumption was that you were accessing this from a normal application, but that assumption was wrong.
Considering you don't have access to the source code I don't really see how you can work around this. Perhaps you can pass this information back to the developers and ask them to implement this properly so you can get it working your end?
Another idea worth trying is to derive a class from this control, and handle the automation event there, and then test this derived class. The only purpose of the derived class would be to add the missing automation stuff. May not work, but worth a shot.