Longtime users of Linux and most other forms of Unix (especially those which used to have startup frameworks based on the venerable SysV init) supported a special file /etc/issue which was displayed on local consoles and serial terminals prior to the login: prompt.
I needed to do something like this for my OpenBSD portable jump hosts. I take these systems to events remotely maintain some in wiring closets (they're in the NUC form factor, not much larger than a pack of cigarettes). But I want quick, easy and unauthenticated access to the MAC and IP (DHCP) addresses (on just em0).
However, OpenBSD doesn't support /etc/issue ... so I had to hack around this with an rc.local and a gettytab.template. (Posted as an answer below).
Question: is there a better way to do this?