It is said that B&W is tolerant of 2 to 3 stops of overexposure anyway. Combine that with a speed reducing dev such as Perceptol which brings HP5+ down to EI 200 and in theory you're in with a fighting chance of producing reasonable negs which will be printable.

Best of luck. If you have ever considered posting to the technical gallery, it would be nice to see how the negs print.