This site has the schematic for the remote on it:
CE-RH1 Joypad. The page is in japanese, but the schematic is still perfectly understandable.
Be warned, make sure you get GND and KEY on the headphone jack the right way around! I didn't the first time, because I assumed GND was the sleeve when it wasn't. If you get weird clicking noises in your headphones when you press buttons it's because you got these mixed up.
Also, take note of how much resistance your switches have. Mine were about 50ohms, which ended up ok except for the Play button! Since that switch only has 120ohms connected to it the 50 extra knocked it way out and the zaurus wouldn't recognise it. So lower the value of that one resistor accordingly.
Oh, and you *need* a hold switch