This is the plan. I say plan... See it more as a brainfart that requires ALOT of actual thought:
An Arduino Uno will receive input from the Keyboard/Mouse (either using Host Shield or using PS/2 Devices and the PS/2 library.
The Uno will also act as an i2c master.
An Arduino Pro Micro clone (based on the Atmega32u4 chip) will be added as a slave for each Machine that needs controlling.
Each slave will carry the code to provide key strokes for all platforms (Win, Mac, Android etc plus custom). A switch added to the slave can determine mode.
The master will monitor the PS2 inputs and redirect the keystokes and mouse data to the relative slave when a key combo is entered; The Pro Micros will be recognised as HID devices and pass the received input to the machine.
In order to make this modular I am keen to design a case that features a backplane of sorts that carries the i2c lines, and then the Pro Micros can be connected by simply plugging into this backplane. This will all the switch to be easily expandable.