I forgot to follow up on this - I did finally find it but only after asking the LH checkin agent where to find it.

I clearly don't know my way around FRA landside well enough. It is indeed on the mezzanine (not departures) level, but there is a separate area on Level 3 which can only be accessed from deeper within the departures hall than the ticket counters. You have to start in the B hall and go all the way back (towards the runways) and turn left to find the escalator with the sign for Terminal 2 (doesn't even say "via SkyTrain"). Go up to Level 3 and then turn right to find the Bundespolizei office there. It is not in the hut in A departures where it used to be; it has moved.

Registration was quick and painless, but I did not attempt to use it on my way out -- officer said "one to two hours" and I didn't feel like cutting it close.