Ok. Thanks for the nod to the wiki. I never used pacman hooks. Seems I misread informants github page. So the alpm conform way would be to copy (and not symlink) /usr/share/libalpm/00-informant.hook to /etc/pacman.d/hooks only if I would want to override it and other hooks are by default read from the /usr dir.
The root of my problem was, that the hook is fired only after Pacman asks “Proceed with installation [Y/N]” and the user presses “Y”. Which I hadn’t tried. DOH!