As Wes says, if painted, MDO would be better. If it needs to be solid wood, I would consider using the same method used for building large wooden rudders for boats. Fasten them together with drifts-long rods that go all the way through. For this application I would use bronze rod threaded at the ends for bronze nuts. At the top, I would use a large bronze washer bedded with a polysulfide type caulk to keep water out of the holes. Since the boards would be hanging horizontally, I would make the joints so they would shed water, The top edge would have a slight crown or slope and the bttom edge would mate to the top.
The reason for bronze instead of steel or iron is that it won't corrode as rapidly and it won't end up with the black iron stains. Quartersawn lumber is a good idea. White oak would be very durable but it is high in tannin which will react with iron.
Irony: The opposite of Wrinkly