Right now it seems that only an absolute path is saved for the extended help .xaml file in the .cuix. Even if you enter a relative path to the file (e.g. %appdata%\Autodesk\ApplicationPlugins\test.bundle\Contents\Resources\tooltips.XAML) it is converted to the absolute path. This obviously makes it tough to include in a plug-in stored in the user folders.
It is obviously possible to add extended tooltips to the ribbon with the API after the plug-in (i.e. .net DLL) is loaded, but that seems rather kludgy.