ContainsDataFlavor for client side validation on custom attribute.

We want to implement client side validation of Transferable on custom flavour, for instance CSS class name of Tree node.
Out of our experience with ContainsDataFlavor, it does not work with neither Tree item attribute, beside “itemId”.
Are there any chances, that setting custom flavor, like CSS class name would be possible for Transferables, made from Tree?

If it was not planned to support custom attributes, may be it worth to mention about limitations of implementation in documentation?