While I firmly believe the wing is easily defined as a sail, the DoG does say specifically "propelled by sails only". So if the wing is NOT a sail, it wouldn't be Deed Legal. It would be different if the Deed specifies propelled by wind only - but it does not.

The challenge also specifies a sloop rig...so the wing has to meet that requirement for the challenge to be valid as well. They can string headsails up and have done so testing in San Diego.