Following Adobe documentation it says that "Internal (granite:InternalArea) - Defines a node as internal. Nodes classified as internal cannot be overlaid, inherited, or used directly. These nodes are meant only for internal functionality of AEM" . This means that you should not modify areas that your product vendor has marked as InternalArea. This has been done to ease Upgrade for your client, this path should not be overlaid.
Thanks a lot for your worthful response. I totally agree that this should be addressed via CR at Adobe. The downside of this will be the time. If Adobe accepts the change, it will take quite a time, until it will be available. And as it is a CR, it might be possible that they only will introduce the new feature in the upcoming (minor) release.
We'll see how important this feature will be for our customer and then decide, wich option we'll choose.