The omnigo has a lower resolution screen, and runs geos by default. It uses the same serial cable. It can do textmode dos stuff, but it takes digging through archives of German websites to find the appropriate software.
I'm working on putting together a complete toolkit with instructions but that'll be a few weeks down the road.
The serial cable, though, is pretty easy. You can rig one up with some jumper wires and some electrical tape.
Eventually, I'll get a pair of calipers after one of mine, and post the exact dimensions (and possibly an STL) for 3D printing your own.
Until then, the pinout is pretty easy to track down, and the pins are the exact width of standard jumper wires.