I've never been a fan of getting the body of a guitar signed. If I had the chance for Mark to sign mine, I'd ask him to put it on the back of the headstock where it's less likely to be worn off and doesn't affect the look of the guitar.
In this case, since the signatures are there, I would seek professional advice from a luthier before I tried to use T-Cut on a nitro finish in case it ruins it completely.
My luthier has a '60's strat that he has sanded or cut a black respray off to expose the original Fiesta red lacquer, but I don't know what he used, and last time I saw it the finish was still more satin than gloss.
