I found the following statement in the „File System Programming Guide“:
„Apps create files and directories in iCloud container directories in exactly the same way as they create local files and directories. And all the file’s attributes are saved, if they add extended attributes to a file, those attributes are copied to iCloud and to the user's other devices too.“
Obviously that is not the case as I found my attribute as well as „com.apple.ResourceFork“ being stripped during iCloud synching to other devices. Other developers are observing the same behavior, here is a well-describing article about the problem:
I've invested a DTS credit, but so far haven't received a response...