Good points. I could see either choice being confusing. The titlebar usually shows the source path, but even that can be edited.
It would also probably need to be turned off when customizing toolbars, else it'd be too easy to start creating stray copies of items you were trying to move around on the menu bar (top toolbar) if you weren't careful.
(I'm also not sure if would require a lot of refactoring, since currently each tree and file display handles its own drop target. If the whole top-level window became a drop target, that logic may need to be moved into a common place which negotiates with the child windows. Or maybe not. I'm not actually sure Windows handles multiple overlapping drop targets, without doing some research. I know it causes problems with some UI frameworks.)
If you're copying things between two top-level Opus windows, when the second usually isn't visible, you can use
Copy to=ask to ask for a destination, which includes a pop-up list of currently open folders in other Opus windows.
You can also drop on to the folder tabs at the bottom of the window, if you want a drop target that's nearer a window edge.