I have finally got my hands on a Pro Micro to start playing with (I got impatient so pulled this one out of an old project (a keyboard emulating foot controller for music recording), and will start coding the slave and some of the master tomorrow :-)
For the master Uno I have opted for a clone. Partly due to a love for it's colour (ha ha), but mostly because it has a micro USB port rather than the Printer-style USB. This way the end user is able to program the master using the same cable as the slave is connected to the target with.
I have had an idea for a further use for the master - I often use a small netbook for coding and suchlike, and it would be great to use it's keyboard as an input to other devices when quickly prototyping.
My idea is to use the serial input on the master as an alternative to the keyboard/mouse inputs on the board.
Up until now in my heads the master's USB port will grant the user with a serial menu to configure the KM switch in a variety of ways. I now plan on adding a menu option to allow 'through-typing' - anything typed into the serial window while in that mode will be passed to the selected slave as in normal use. a pre-defined keyboard shortcut would end this mode.