Les Minimes or the bassins in the Old Town are about the only options, aren't they? Saint Martin de Ré and Ars en Ré are gorgeous but I rather suspect moorings are handed down from father to son
I think you can get a berth at Minimes if you agree to vacate it for a month I think in August or September not shure which. You probably won't get a berth in the Bassins and as jhr says you can forget about Isle De Re'.
Les Minimes will only take winter bookings after the Boat Show ends in September, so should be OK to call now. You'll need a bike though as it's some 3 miles to town. Worth a try at the locked-in Old Basin in town - noisy, but you're right in the heart of the city, which is delightful.
Friends of mine managed to get a berth in St Martin (I de Re) a couple of years back - cosy and well packed in (don't expect to sail over the winter!), but friendly and a nice place. Need a bike though.
Otherwise Rochefort is v popular for leaving the boat unattended as there is a good liveaboard community who will happily keep an eye out for your boat.