Thanks for the advice guys, I practiced it all today's session, and have the following to report:
I did one!! YEEHAA!! This was almost at the end of the session, and I couldn't replicate it, but now I know it's possible.
The key seems to be keeping the feet near the centerline, and getting the old forward foot back down the board once you get around the sail. Also I had real trouble keeping the sail up and out of the water, so many times I'd get the board around but only by hanging onto the uphaul rope as it dragged in the drink. With another couple of sessions I'll have it sorted.
Practice is the other key