posted this elsewhere before, but the now-defunct RPG company, Skorched 'Urf Studios, has 2 PDF's to emulate Mega- damage in OGL/Pathfinder-based games, titled "Military Scale Weapons" and "Military Scale Magic". Get them where ya buy pdf's, but the two pdf's are about as close as doing MDC in OGL as you can get. You can use the MS Magic pdf for psi, like the Pathfinder 3PP stuff.
