Yes, proton therapy can be used as a salvage treatment. Shown below is a link that includes testimonials from three men who have gone that route.
Personally, I considered PBT as a primary treatment for my PCa but ultimately rejected it because most of the "evidence" was anecdotal, i.e. I could not find long term studies that proved its' efficacy.
If you want to seriously consider this option, I suggest you call one of the PBT centers and speak with one of their physicians about your case.