Guide to the Macintosh Family Hardware marks VIA1, data register A pins 0-2 as "reserved" for the Macintosh II, IIx, and SE/30. At least on my IIx, these pins go to the diagnostic edge connector, and appear to be pulled up to +5V. They don't seem to go anywhere else, so driving them as outputs doesn't seem like it would hurt anything.
Nice! My checks show that pin 7 on the IIci logic board's edge connector is connected to VIA1 pin 2, and your control panel correctly controls it. Just like on the IIx, it's pulled up to 5V with a 100k resistor.
That's interesting, I had heard that before, but hadn't looked into it. Do you have any details on that? The IIsi ROM has the netBOOT/ATBOOT driver in it, but that driver didn't exist prior to the IIsi ROM, and that driver relies on PRAM settings for the server to request, file, file checksum, etc. Those drivers have an ignore bit set in the resource map attribute, so the resource manager usually doesn't load them. It seems reasonable that keying off of something like a VIA input could enable it. I haven't come across anything in ROMs prior to the IIsi that indicate a localtalk booting ability. I haven't spent a tremendous amount of time with pre-IIsi ROMs, so it's entirely possible I missed something.
dougg3, techfury90, and I were looking at this last night. It turns out it actually enables a diagnostic function on the modem port, which presumably is what the techstep devices and the factory used during burn-in testing. It's a mostly ASCII protocol too.