Re: "is locked?"
 

After panicking and trying several run-throughs of "surgeries" to try and revive the black (not blue like a "wrong firmware" player; no light at all), I decided to try it in the USB jack I hadn't used yet. Voila. Same firmware + old dump used before, same "is locked" screen (so this problem still isn't technically solved, but given the alternative of no working player at all, I can live with that).

arfies, Good to hear it :)