The sshd_config banner directive reads a file, but I want it to run a script, and ideally personalize the banner to the user... is this possible with ssh, or is there a way I can create a device file or fifo that runs a script, then have sshd read that for the banner.
sshd is able to do this with directives such as AuthorizedKeysCommand, and pass details about the user to the script, so there is some precedent. Also the daemon at least knows the username attempting to connect before displaying the banner.