Thanks for all the helpful ideas, all! As it turns out, the solution to my problem was straightforward: I needed to do a dpkg-reconfigure mdadm, and specify that all multi-drive arrays need to be started in order to access root. Once that was completed, update-initramfs produced an initrd.img which permits booting on my computer. Like the man said about sharpening and aligning a plane, "None of this is rocket science, but at the same time you're not born knowing it!"
-PT
