Someone (I don't remember who did what here) discovered that they could get `SHOW SYSTEM` (roughly analogous to unix `ps` command) to display their name in reverse video by adding escape sequences to their process name. So a bunch of us started experimenting to see what else we could embed in there.
Most of the terminals attached to the VAX were Zenith Z-19s, which mostly emulated DEC VT-52s but with some added features. One of those added features was an enablable 25th line (in addition to the regular 24x80 display) that functioned as a sort of status line. We found we could enable that, write something into it, then use the "transmit 25th line" escape sequence to send its contents back to the VAX. I remember having to work around limitations like it sending an escape sequence before the 25th line (which confused VMS), and I think it didn't send a carriage return at the end... or something like that.
I don't think we ever got it to do anything terribly interesting, but it was fun to play with. And then IIRC a VMS update blocked control characters in the `SHOW SYSTEM` listing.