In this particular situation I'd suggest making the extra term a
non-binding request, something like:
The author of this software requests that you retain the iText producer
line in every PDF that is created or manipulated using iText.
or even:
The author of this software requests [...etc...] to facilitate debugging
of any post-processing issues and [... other reasons? ...]
or even just insert similar wording as a comment in the source code, next to
the code that adds the producer line. Anyone who's not being deliberately
awkward will probably comply with your request, particularly if the reasons
are as compelling as you imply, and if they *are* being deliberately awkward,
perhaps they don't deserve your help :-)