OK, I think I got proof-of-concept code working, but it does not yet have a GUI.
This is how it will (should

) work:
You can select which forum shold be cloned.
Then there is a Textarea-Field to input the new forum names (1 per line).
The script will the copy this forum (and all its subforums) as often as necessary and name it accordingly.
Optionally, it can also clone moderators, forum permissions and access masks.
It does (of course) only copy the structure, not the contents.