If he did not take an oath to the Bourbons it was because they wouldn't have him, not because he refused. I'm sure there is an earlier letter (which I can't find at the moment) in which he requested royal approval. He definitely accepted the King's authority.
This business of taking an oath: I know dignitaries swore an oath to the king at a coronation but did they, or military officers in general, actually swear an oath on taking up their post? This is something I'm not sure about. I don't remember anything about any of the marshals actually formally swearing-in in 1814. The bit always mentioned is them submitting their adhesion to the Provisional Government. I would expect members of the Chamber of Peers to have sworn an oath on taking their seats for the first time but I don't know about anyone else.