Okay, lets get some stuff moving! I need a software man!

Since Sailfish has the I2C_HID driver built into the kernel and the TCA8424 is I2C_HID compliant, it should be do-able to connect the dots.

I imagine the phone wants to know that:
-what address the chip is on
-the chip is a keyboard
-it should listen to the interrupt line and then read out the key
-key-mapping in the phone? (there is a keyboard mapped on the chip)
-pushing the input to Mallit