After finally being able to fix Debian bug #794266 I want to thank those who made this possible:
Some time ago my colleague Bjørn offered an Arietta G25 to me. After Jochen, another colleague, helped me to solder pin headers on it, this machine served as host computer for my tests.
As I didn't have a machine with the relevant RTC chip, I contacted Seiko Instruments and they provided me a few chips including oscillators. It was again Jochen who then created a break-out board from these components that I could wire to my Arietta board.
Finally Wolfram Sang's i2ctransfer helped me a lot to access and so understand the chip. It's has not landed in i2c-tools.git, but I hope this will change soon given that this is a really useful tool.
A big thank you to all who helped me. It was fun and would have been less so without your efforts!