Thanks, Dave, for the answer.
A slightly more difficult change would be to add an optional primitive for Unix/Linux that sets the serialPortBaseName string. That would let you control it from Squeak, and would not require any changes to the existing primitives.
That's actually probably what I'll try to do. Stupid question, but where can I fetch the sqUnixSerial.c file (and it's MacOS counterpart)?
Severin