The HP iPAQ 210, 211, 212, 214 do have a serial port. Unfortunately it's not connected to the 24-pin connector at on the bottom. After lots of searching I eventually found the serial port on some solder pads inside the iPAQ. I now use it for my work porting linux to the device.

Where to attach the wires to the iPAQ mainboard:

Originally, I put some flat stiff wires in routed past the battery and around the case:

The circuit used a transistor to convert the 3v logic to 5v and then a MAX232 for converting that to the RS232 levels:

I then put it on a veroboard:

Later on, I put a mini-minijack in the side of the back case and connect it with enameled wire. I'll take some photos of that one day.