As I mentioned, it is work in progress.
Started with arduino and made it work.
Later, when my attiny85's arrived I did the attiny part.
Made updates to get that working but did not update the arduino slave accordingly.
And on attiny focused on getting the servo working which needed the regular amount of hacking and debugging.
I think part of that debugging is still in the example, i2c_expander_master and both slaves.