Sean
Parkers do sell a lock down device for the 275. The idea would be to stop the keel starting to move the wrong way if you were rolled over.
I doubt if you were unlucky enough to be rolled by a wave that you would be unlucky enough to end up upside down!
I have managed to get the mast fairly far down by tying figure of eight knots in my spinnaker sheets when I first got a spinnaker. Not advised - you don't want to hear the whole story!
I have sailed Speedwell a long way including around the Raz de Sein etc and have never even thought about the issue - haven't got a lock down system. I have in a good force five come back across the channel with the keel half up to try to get home quicker! I suppose at the end of the day the best thing to do is try to avoid the worst conditions.
Tim (Speedwell 275)