Thank you for the suggestion!
Okay, I've successfully registered the ClassicExplorer32.dll and rebooted.
I did some more tests and this it what I've found:
Drag-Drop from almost any x86 or x64 application into 64-bit Windows Explorer will bring up the Classic Shell Copy dialog as expected:
However, Drag-Drop from any x86 or x64 application into any x86 application or any x86/x64 Open/Save dialog box will bring up the default Windows Copy dialog:
Is it possible to somehow intercept *all* copy/replace requests and invoke the Classic Shell Copy?
Now that I'm seeing the default OS Copy dialog more often (due to using XYplorer) I'm really struggling to make decisions whenever prompted by Copy/Replace... it's such a confusing design!