Do you think you could add a balloon QML control?

It's the kind of thing that appears as a notification, not when the item is hovered. In some cases it may have an icon, or a close button. I know it's supported natively by Windows.

e.g.

http://social.msdn.microsoft.com/Forums/getfile/117300

Windows has such a balloon for the System Tray:

The UI developer would be able to use such balloons to mention invalid characters in a TextField, to notify the user when something happened, or to provide a quick tutorial / intro on how to use the UI interface.