So the PAM module which I pondered about in my previous post now exists:
I did mention "sponsorship" in my post which lead to a couple of emails, and the end result of that was that a couple of folk donated to charity in my/its name. Good enough.
Perhaps in the future I'll explore patreon/similar, but I don't feel very in-demand so I'll avoid it for the moment.
Anyway I guess it should be Debian-packaged for neatness, but I'll resist for the moment.