After finally being able to fix Debian bug #794266 I want to thank those who made this possible:
Some time ago a colleague offered an Arietta G25 to me. After 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. The same colleague as above 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!