aiui, the point here is to have the user "service" get triggered somehow (through pam's pam_systemd.so's session module?) before ssh goes ahead and forms the socket. is that right? If the pre-launch mechanism is pam, is there a reason to do it as a systemd user service? That won't work for systems that have pam but don't have systemd, whereas a pam module that creates these will work.
