# rm -rf /mnt/*
I then rebooted back into the ISO, and this time mounted the EFI partition to /mnt/boot before performing a standard install, including reinstalling systemd-boot. When I booted the new install, the keyboard worked fine.
I think what was happening is that when I performed the full system upgrade, the system wanted to upgrade the kernel, so it upgraded the kernel in /boot. Problem was, that wasn't where the kernel I was actually using was located; the real kernel was in the EFI partition. I'll admit I'm not totally sure why that would cause the specific error I encountered, but it sounds pretty good, and my problem is fixed now.
]]>pacman -Syyuu
(I did it this way in order to solve an issue with Firefox and glib). After the next reboot, my keyboard no longer works at the login shell. I can select boot OS with the keyboard in the boot loader just fine, but when it hits the login prompt, typing no longer does anything.
Note that I still have my bootable USB, so I can modify configs and such from that.