I do not believe GroupWise exposes any drag and drop event handling, either via the Object API, Token API or any other mechanism. This is certainly an ommission. Unfortunately Formativ is only able to take advantage of the underlying features GroupWise actually exposes.
My understanding is that the GroupWise XIS API _may_ provide change event handling. However, if and when this API is made available is unknown at this stage.
This only leaves the TPH API. As you have written a TPH DLL to trap all tokens, you should be able to determine if this is going to provide at least some of the functionality you require.