OK, so more "grist to the mill".
We decided that in our modelling environment, we would allow ControlFlows to be bidirectional. Since the UI disabled the direction property of the ControlFlow, we changed the value directly in the t_connector table. Unsurprisingly, the direction property was still disabled, but what was surprising was that the value displayed in the dialog was still "Source -> Destination" not Bi-Directional! Imagine our further surprise when the shapescript we updated DID show the bidirectional addition, correctly responding to the Source shape's:
EAUI of the month award!
So, given what we've found, I think I should add a feature request to allow ControlFlow direction to be altered. (We're not interested in exchange with other than our modelling environment at this stage)