To continue the sad tradition of answering my own questions - I should really learn to sleep on things before firing questions into this forum ...
The magic incantation is ...
At the OS/2 command line: MODE COM1:9600,N,8,1,TO=OFF,XON=OFF,IDSR=OFF,ODSR=OFF,OCTS=OFF,DTR=ON,RTS=ON
Set the terminal...