The tech at UD identified the relevant file as /var/isy/FILES/CONF/0.UCF . It's binary, so changing it isn't trivial, but it's a very short file and it's doable. I have my secure eisy port set to the value I want now. Thanks, all.
Problem turned out to be the PLM.
UD clued me in to the existence of the "Diagnostics / PLM Info/Status" function, which reported zeroes. Replaced the PLM with my new-in-box unit, which also showed zeroes at first... but shortly woke up and began throwing links around. I ran the Restore PLM function and all seems to be fine now.
I would assume the cable subsystem on the old PLM failed, as its connection to the other Insteon devices was still good (as shown by the triple tap test).
US also clued me in that Reboot and Restart are two different buttons, and that I needed to be using Reboot after a software upgrade, which I wasn't.
Thanks for your help, everybody!
