Thanks guys for the responses. I did change the QR today and it also looked to me like the Derailleur hanger is some fairly hard steel, not giving the QR nut a chance to dig in. I ground out the area of the hanger where the nut sits a small amount and filled it with brass, then filed off the excess. While I was at it I brazed up some nubs at the opening of the QR dropout that prevent the nut from sliding forward at all when it's tight. The sucker ain't goin nowhere now!