I've solved, for details here: https://community.plone.org/t/experience-with-shibboleth-and-autousermakerpasplugin/7900/6
Basically, zope/plone removes the X_ request environment headers. Just use a different name (not starting with X_ which was my default...) and it worked.
A great plugin, easy to configure/install (uninstalls cleanly), and do what you would expect from it.