It would be so much easier if you knew much more about object-oriented programming.
Looking at it from a workflow point of view:
If an object would pass on a dragged object to the next entity in a GUI, e.g. the menu bar which would not accept it. So the menu bar passes it on to the filter box which cannot deal with it so it passes it on to the files list which detects an unsupported file and so passes it on to the tag panel.
And like that the object gets passed around and MP3tag simply stops working. And you do not get any user feedback that you just tried to d&d an illegal object into MP3tag.
So I would say: let us keep it as safe as it is with a decent user-action-feedback. I am sure you will be able to hit the biggest area in MP3tag.
Of if that is not feasible, use the "open folder" function to get to the file.
Or copy&paste the path from the explorer window into the tag panel directory box and press enter.
How come that you always have such difficulties in handling the GUI?
Grabbing the tag panel, for instance: /t/18620/1
Using menu functions: /t/16741/1
Using custom menus: /t/18380/1
Edit: as the whole scenario is yet again very much dependent on the favoured workflow, here is one where "dropping anywhere" is plain dangerous:
You edit using the extended tags dialogue - an application modal dialogue.
For some reason, you switch to the explorer and navigate to some other files, the explorer window hides the extended tags dialogue.
If you now d&d files from the explorer to MP3tag - and here even to the files list - you get the blocking cursor. ANd quite right so.
Adding/replacing the new files without finishing the current edit would lead to some really unexpected results.