Okay crazy. I2C has some additional rules

The OH could act as master and phone as slave, or -new possibility!- both could act as MultiMaster (I like how this sounds very much!) But as you said, this totally depends on the phone and its kernel.

If the phone can act as slave and/or multimaster, we are done! Interrupt lines are for noobs anyway

If the phone can act as multimaster, we can start looking for a multi-master keyboard controller. If the phone can act as a slave, it will still work with multi-master keyboard.

Please correct me if I am wrong, this is new territory!