AIUI in many jurisdictions e.g. EU, copyrights can not be waived, rights must be granted.
If this is the case, then "This file is in the public domain and may be freely used for any purpose." would likely be sufficient to explicitly grant those rights to us.
Sure,
but these other people are already downloading many public-domain files
from us and are are using our code and scripts to munge them
A fair point; all we need are the rights to distribute it, and since the above is a superset of the permissions we grant when passing along our files, it introduces no other issue.