Yep, I'm not buying the Lord Octesian angle...from a very practical aspect, it would just detract from the Eustace storyline and confuse the crud out of people.
Here's my completely personal, non-verified hunch on what's going on.
Eustace gets turned into a dragon. Not being the brightest kid on the block and perhaps not thinking through what kind of reaction the DT crew would have to his appearance, he flies to the ship and is very surprised and taken aback when he's fired at and injured. Disappointed and a sadder but wiser dragon, he flies back to the Island.
In the meantime, Lucy got a good look at the Dragon while he was at the ship and can't shake the feeling that something is up. She recognizes that he wasn't trying to attack them, and feels awful that he was injured. She convinces Caspian and Edmund to put to shore so she can try and heal the dragon's wounds. There, they meet the dragon face to face and Lucy is able to discern that he is, in fact, Eustace.
That's just my completely random opinion.
Don't ask me why, but it is. Does it sound plausible to you? It would seem very in character for Lucy to be concerned about the dragon; she was always the one who could discern the motives of people and she would have seen that Eustace-dragon was not trying to attack them, and as the compassionate Healer, she would want to right the wrong done.