Oh my freakin' God. I was right about it being something funny.
So, after I removed the USB 2.0 Powered Hub (as mentioned in the comments), the light and optical drive turned off. My machine was truly removed from any source of energy. I then swapped out my old surge protector for a new one (the brown-out alone is a perfectly good cue that I needed a new one anyway), plugged everything in, flipped my power supply main switch to off and held my power-on button down for sixty seconds to drain the caps (I know thirty was more than enough, but I wasn't taking chances with what was on that drive), flipped the power supply switch to on, and powered it back up.
Everything booted smoothly, and the drive-in-question (and all of my work) was visible again. I'm going to run a hardware check on the Linux side to be sure, and probably invest in some active online backup; but my best guess is that during the brown-out, my drive did not properly shut down, I was running Linux, and it is where my Linux root partition is. This may not have been an issue if it was truly powered down, but that USB back-power may have been tricking the drive into thinking that it was still powered on. Thus, for one reason or another, UEFI didn't detect it as a valid drive (or even one that physically existed).
That is largely speculation, but it fits the data. USB hubs are finicky creatures. Peace!