Originally posted by GunsForBucks:

Can you hang a head without killing the boss?
If so you could bring the head from another world and hang it in the new one for the buff while still leaving the world level low.

Yes, the source of the head is not important. However I am not sure if hanging the head registers the world level or not. I am not sure the world level still works that way; I am getting hard hit by swamp invasions and I just got there, not even found the boss marker yet.