Simple as changing the cord
Ok, that cord that i showed in the last post? Either mod it or make an adapter for a 4 contact jack. Below is the picture of what i mean.
All you have to do is mod that cord or create a jack at the end. Uart is only a 4 wire interface. You dont need to use the RTS or CLS lines. But you do have to either set them to ground or power, i forget which.
But im pretty sure it will work like this. However, this will handle the hardware problem. There still needs for a driver to be written. But with so many usb keyboards, i dont see that being a problem.