This is a bit off topic, but something I've found really interesting. I picked up a clone of the Sparkfun Pro Micro which is a fancied up Arduino Micro. It was around $15 for a pair of them. I bought it to build a "mouse jiggler" (it can look like an HID on the USB port, so you can emulate a mouse or keyboard).
With a simple program (less than a dozen lines of code), this could act as a TTL level (it can work at either 5 or 3.3 volts) serial to USB interface. It has 12 other digital I/O pins and supports SPI and I2C.
If you did use it as a serial to USB converter, it seems like it should be possible to intercept the serial stream and do something with it. Without some type of off-board storage, it would be limited, but with an EEPROM connected to the SPI interface it seems like you could potentially have a few images that would load into RAM. In theory, you could also use some of the outputs to automate things like switching on the on-board keyboard and LEDs or switching ROM banks.
The fact that you can get something that adds a reasonable amount of processing power (by 1980s standards) in the "serial interface" for a few dollars is pretty cool. I need to spend some time learning more about SPI to figure out how usable that would be.