Thanks for your prompt response Nick. That all seems quite logical.
However, I'm not sure your car analogy really fits here. While my car has brakes, the "brake" on a boat is throwing it into reverse. I certainly wouldn't want to blow up my esc. (In fact I'm still gathering the kit together as I haven't used brushless motors before, so I don't actually have the bits yet for even a controlled bench-test.)
Since the brain (such as it is!) is more readily available than the cash at the moment, I guess I need to drag out my thinking cap from the bottom of the scraps box and dust it off.
Electronically I suppose I'd monitor the signals in the three power leads and select the appropriate moment (presumably when there are no signals) to do the AB - BA swap. I guess this is something like the way reversing ESC's work anyway, so I needn't try to re-invent the wheel.
[Going off at a tangent, did you read about the guy who built a £4.50 (about A$8) toaster from scratch - ie metal ores, crude oil etc. It took him six months, cost £2000 (labour not included) and blew up after thirty seconds operation!]
Mechanically, I guess I could rig up an interlock that allows Andrew's reversing microswitch(es) to throw only when the propshaft is stationary. Of course it would have to work both ways - ie from backwards to forwards as well. Maybe it would be better to stick with a one-way only motor and hang a couple of reversing buckets off the Kort rudder.
For my next tug (even though I've barely started this one) I fancy fitting an asimuth drive. Adam's setup in Reply #10 of Brushless Motors in Tugs -
http://www.modelboatmayhem.co.uk/forum/index.php?topic=22743.0 - looks really neat.