I saw somewhere (shoot I can't remember what site it was...) where someone has this exact same problem.
I do remember, however, how they resolved it. They actually ended up filing away the part on the seatstay where the chain rubbed in order to use that last sprocket.
However, I have also read that some people simply just lived without the use of that last sprocket.
I would be hesitant to file away part of my seatstay, personally, but if you have already cold-set the rear triangle, filed away at the rear dropouts, why the heck not? File away the seatstay.