Well, Meera and Jojen know it. That doesn't mean its well known in the Neck. Thats possible. Although he was too shy to leave his bench. But it does not specifically say Ned had any interest in Ashara, only that Brandon asked Ashara to dance with Ned. 'On behalf of' can mean 'at the request of' or can mean 'for the good of'.  Also possible is that Ned had no interest in Ashara, but Brandon decided he was missing out on the party and got Ashara to ask Ned to dance, which he couldn't