You can use key->OSC->MIDI for what you want to achieve. I don't know if you can install max runtime on the other computers. That would be the easiest way to communicate with other computers through OSC. See the example code below.

Thanks ahmet, that looks interesting. I will see what I can do, because one Mac is a G4 with Max 4.6 the other a G5 with Max 5 and two Intel based Macs. Cheers