Currently, it is impossible to allow a user to drag and drop a game object from your place into a plugin GUI. I want to create a plugin that will allow users to drag objects from the explorer window into a box in the plugin widget, but PluginDragDropped doesn’t fire and I just get a 
symbol when I try:
The only alternative solution would be to make the user first click a button to start object selection, select the object in the explorer, click “Confirm” on the plugin GUI, then get the selected object using Selection service. This method requires 2 more steps on the user’s end. It would slow the productivity of my users if they have to drag and drop many items (not to mention it also seems like a bit of a hacky approach).
Sidenote - allowing this behavior could also open up the ability for the properties window to accept dragged objects as object values.