Actually, never mind: it seems that for some reason my cryptsetup package (which provides the initramfs-tools hook script) was out of date. After updating, and rebuilding my initrd again, I got a message that indicated that it was looking for a target named sda5_crypt once again, so presumably if I had updated cryptsetup earlier on, I wouldn't have had to mess around with the script and with crypttab.
Sorry for the noise.
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