Yes, this undocumented feature is present in WD9. As it usually happens with undocumented features, it is implemented with a special object name.
Use following name:

htmlclass=MyClass

, where "MyClass" is the class you want to add to the object.

Basically, it's the same syntax as with "filename=".

Make sure you know what you are doing when using this feature. It may not be safe, so if you get unpredictable results, revert back. Also feel free to ask if you have any problems with it.