Thanks Tyler.
I've already tried switching the drive's logic card with that from a known working (identical) drive... no luck.
But...
With it hooked up to a system with a more recent host adapter, and keeping it "on ice" (freezer gel packs), it appears to be a bit more interested in running than it was previously. Hopefully I can continue to coax it along for at least as long as it takes to copy everything to the new drive.