Hmmm I know what you mean - I would probably be cautious too about putting this on hair that's looking damaged. Have you tried purple shampoo/conditioner? That can really help brassiness. John Frieda has one you might investigate: http://www.johnfrieda.ca/en-CA/ProductDetail/Hair-Care/Sheer-Blonde/Colour-Renew-Tone-Correcting-Shampoo It's in the Sheer Blonde line but might still work for your highlights.
Otherwise I'd probably go back to the salon, maybe they can put in a toner? Probably essentially the same thing as this L'Oreal product but you'd be under supervision in case anything goes wrong :-)