[SOLVED]:
The file to investigate & edit is:
/usr/local/etc/inc/authgui.inc
Must restart webserver after editing for the changes to appear:
/usr/local/etc/rc.restart_webgui
NB: this almost certainly gets written over during major OPNsense upgrades, so be sure to keep backups of any customizations & plan to re-implement them post ugprade.
Where are the file(s) that create the user / password login banner for the web gui? I.e. the landing page when you go to http://192.168.1.1/:80 (http://192.168.1.1/:80) or :443 (or whatever the instance's IP address is?)
Since I use SSH -L 9090:localhost:80 tunnels to connect to my various OPNsense instances I don't see the actual hostname or IP address in my browser nav bar. Thus I get easily get confused as to which instance I'm logging in to.
By putting a prominent host identifier right in the web-gui login page I can avoid this confusion.