So I carried on taking apart intil I had a pile of parts: three DC motors, some opto-slot sensors, and the scanner head. The latter looked like a sealed unit, with no mirrors, cold-cathode lamps or anything re-useable. But closer inspection revealed a 220mm long RGB LED lighting unit, easily desolderable. So that's now ready to be hooked up to the Arduino! I already have a simple sketch that takes polar co-ordinate input from a resistive touch-screen and generates colours with PWM on an RGB LED.
Right next to the RGB LED gadget, there's an amazing array of tiny lenses (or light-guides, or something) and beneath that, there's a PCB with a long strip of silicon photo-sensors on it. Just bare silicon, about 0.5mm wide and 220mm long, although it looks like there are joins where it's been made up from several smaller chips. All the bond wires are exposed, and there's another (tiny) chip on the PCB, also with bare bond-wires connecting it to the sensor array. This is definitely something to get under the stereo microscope and examine closely!
Apart from the scanner-head stuff, there are a total of three DC motors in the device, and the usual paper-path sensors and a power supply.