The situation with the storage has been resolved, OHM should once again be fully functional.
I neglected to copy over the BIOS boot partition from the good drive over to the replacement; thus, GRUB was unable to boot the system. After sorting that out, the system would boot only as far as its initramfs, owing to a mismatch between the kernel being loaded and the ZFS kernel modules in the initramfs, meaning the zpool couldn't be imported on boot. Since the root FS is on that ZFS pool, this prevented the system booting.