I just spent over an hour on the phone, calling various places in Germany.
It's the usual thing: when you try and talk to a person at a giant corporation like Deutsche Post, all you get, on every phone number they list, is an automated voice menu system. Eventually, I ended up being able to enter the tracking number for the passports, only to end up at a voice version of the same database that I can access over web, which tells me what I already know
It is impossible to talk to a real person at Deutsche Post, apparently. All the phone numbers listed on the web lead to an automated answering service. Searching the German white pages yields a bunch of extra numbers, but they all do the same thing…
So, I started searching German Google for hints, entering various search terms relating to the logistics centre in Frankfurt where the passports are stuck. Eventually, I hit on a forum (much like djembefola.com), only for coin collectors. A guy had posted there with a problem similar to mine, with a shipment that was delayed, no way to reach Deutsche Post, etc. He kindly posted an unlisted telephone number that he had managed to get from someone. (Thank you, Sergey Brin and Larry Page!)
So, I tried that number and, within two minutes, was speaking to a very helpful real person, who passed me on to another helpful real person.
Upshot is: the passports went on a plane on 13 December, beyond a shadow of doubt, and would have arrived in Australia on 15 December.
The problem is that the Germans, once the shipment has left Germany, do not ever again get any kind of information from the Australians. (That's registered mail for you!) Basically, the reverse information channel doesn't exist. For any kind of shipment from Germany to Australia, the last thing the Germans ever hear is that it has left Germany. If the plane crashes on the way over, the Germans don't have a clue it happened.
The information I received from Australia Post was nonsense because all they did was the same thing I did all along: ask the German database, which reports the last known point of the shipment as being in Frankfurt.
So, the passports are in Australia, most likely in Sydney. Because of the Christmas rush, they haven't moved. I'm going to call Australia Post tomorrow when they open and I'll be like the bulldog that won't let go once it has closed its jaws.
There is hope yet!
PS: We should make a habit of posting salient phone numbers for key contacts in the djembe world here too. You never know what desperate soul you may end up saving years in the future