[Ian.M]

It might be better to hack in via the serial port. A simple pulse of the right range of lengths will be decoded as various characters depending on the pulse length. The serial line needs to idle at 1.8V (K3 logic '1' level) and be pulled down to 0V (logic '0') for one bit time to send the start bit of a character. If it immediately returns to '1', the character 0xFF will be received, if it is held low for one more bit time, the character will be 0xFE, two more bit times 0xFD and so on to 0x00 after eight more bit times. If held low for longer it will get a framing error.

To do something useful with this, you would need a keyfob wireless remote kit and receiver + a circuit to generate the correct delay for each button and limit the voltage to the K3 to 1.8V max. It should also force logic 0 if the kindle's TX line isn't idle at logic '1' or pulsing so it doesn't try to apply a signal when the kindle is off.

You would also need to write a daemon to run on the K3 that monitors serial input, looks for characters, counts the '0' bits in them starting from the LSB and fakes key presses as appropriate.

Incidentally, I find it much simpler to read in bed on the PC using a 5 button wireless mouse with a very large font selected, the screen dimmed and one of the extra buttons set up to turn pages, but I do encourage you to pursue this as a simple standalone hardware interface for external buttons would be a real boon for users with physical accessibility issues.