Sadly you are likely to continue to be out of luck. You could get hold of the custome view by casting the view above it in the heirarchy to a view group and getting the child. The custom view though is unlikely to have the information you want. Android and Robotium will not know any details about it other than its view type.
What you can do though is get developers to give you this information via tags. E.g. if it contains the text "hello world" they could also set a tag on that view which would allow you to get the text from it. This sounds like it might not be possible in your situatuion.