Well, I tried a couple more things today (what could I lose after all?)
I popped the cover and dropped in another internal drive-upon rebooting the box went straight into some kind of "HDD file problem" mode and spent the next hour or so looking for errors on the drive. I didn't find any, but still wouldn't boot.
I think went back into the diagnostic menu and was able to get all the way through. The file system test still failed for the internal drive, but the one for the external (I connected it for this go 'round) one passed with flying colors.
I've learned one thing here, the internal drive needs to be functional for the unit to operate even if you have an external connected. (You're not simply bypassing the internal-the box is well aware of what's going on...)
My best guess is that the HDD controller for the internal drive is hosed, although it's a different controller from the one that runs the external. If it were simply a bad drive, popping a new one would have fixed things-