Folder should automatically generate a "Desktop.ini" file.
See "Creating a desktop.ini file" at MSDN.
Windows makes many requests for these files in folders. They are small and can do a couple clever things like assigning an icon and tooltip to a file - and possibly restrict re-sharing.
Just redirecting requests for "desktop.ini" to a "DesktopINIObject" that implements the magick would be simple enough. If the user creates a real desktop.ini it can simply trump the 404 fall-through. So handling this request should be the last step in the object_for_key method.