I think you're worrying too much about the "high grade steel, light weight" issue. There are some great options out there for new frames without those caveats. BTW, I have several bikes built with high grade, light steel that are great frames, but my Salsa Casseroll rides every bit as nice, weighs about the same and cost little for a new frame.

However, if you want a higher quality frame, the Rivendell Betty Foy or Yves Gomez might fit the bill, altho Rivs aren't known for being light weight. The Handsome and Soma frames cost about half as much with tig welding, but probably ride just as nice and weigh about the same.

http://www.rivbike.com/product-p/f-foy.htm

http://www.rivbike.com/product-p/f-gomez.htm

http://www.handsomecycles.com/bicycles/she_devil.php

http://www.somafab.com/archives/product/buena-vista

Mixte frames are a niche product. If you truly want a high quality steel version, you will probably need to order a custom frame or get lucky and find a vintage one for sale in eBay or Craigslist.