crocein scarlet can be used for this. I have a bottle, but I don't think it's available new anymore. I imagine spot-tone could also work. Try painting it on the back of the film. Some large format films have a gelatin layer on the back just for retouching. Of course I should warn you that my last attempt at fixing a pinhole with these methods were not a 100% success. It worked, but you could still see evidence of the spot if you looked closely.