Ive just stumbled on something to help massively with the software for this:

http://www.instructables.com/id/Bluetooth-Keyboard-Mouse-Adapter

This uses an arduino to read PS/2 keyboard and mouse input and sends it straight to a Bluetooth HID dongle for sending to a connected device.

I can use this code as a platform to handle the keyboard and mouse input capture, then modify the output to send to i2c rather than serial.

Yay!