I just knew it couldn't be that easy
I used the master password to set Arco to AI.
The turn ran, and I downloaded my own (R'lyeh) position, and guess what.
CD Key Violation. Your CD key is in use by another installation. The game thinks that Arco and R'lyeh are being played from the same CD.
If this doesn't correct itself next turn (when Arco is established as an AI) then I'm out of the game.
Sigh.