The client-side validation is not implemented yet. Once it is, it will be general enough to work with external drag&drop as well.
Actually, there are three possible validation levels:
- Server-side. That means a roundtrip to the server and possibly showing a message.
- Client-side (after the event is dropped). This can already be done using EventMoveJavaScript (set EventMoveHandling="JavaScript"). Do you validation in the client-side handler and call the server if it's allowed using dpc.eventMoveCallBack().
- Client-side (during drag). This is probably what you are asking for. This one has to be implemented as a new feature.