It looks like I'm not the only person who thinks this is unconscionable; someone on the Austech forums posted this handy unlocker that just needs a 5V serial port (either the official "Bosch" DF266 programming cable, or whatever TTL adapter): https://pastebin.com/ujj2Q6Tg
It looks like a pretty simple challenge-response type thing, unsurprisingly.
Note that the M16C pins *need* a 5V input, a 3.3V UART won't hit Vih