Appreciate the brainstorming!
Intel/AMD (Gigabyte ga-z87x-ud3h). I've tried numerous combinations of plugging/unplugging, different ports, etc, to no effect. I can't find any settings in the bios to truly shut off USB ports, but doesn't seem like it would help, given the results of unplugging/plugging?
Oddly enough, there were a few times it worked at the grub screen; If (and only if) I went from the keyboard -> USB extension cable -> USB port in the side of a monitor -- the bios boot screen worked with the keyboard in that case. But if I plugged keyboard -> extension cable -> [desktop, or USB port in the front of desktop], it wouldn't work until the OS is booted... and at any rate, it was only temporary: at this point it doesn't work at the grub screen in any of those configurations. (And the same is true for both of the A1243 keyboards I now have. so it doesn't appear to be an issue with a particular keyboard.)
Given the intermittent nature of it, I was left wondering if there was just some hardware issue with this model that fails over time, or some kind of bad shielding that let EMI cause problems, etc. Regardless, I can't find a configuration that works. Hence my speculation that some kind of trickery where I use an intermediate device to bridge between the keyboard and the computer might be possible.