Just talked to my mom and she was telling me she found a recipe that almost nails Parasol's roast beef po' boys. Parasol's is an old New Orleans legendary sandwich shop/bar. They were featured on Triple D (Diners, Drive-Ins, & Dives), and somebody watched it and put their method into print form on the interwebs, and that's the one my mom found. So here it is, for any others that want authentic without leaving home.
http://www.nolacuisine.com/2009/07/21/parasols-style-roast-beef-po-boy-recipe/
http://www.nolacuisine.com/2009/07/21/parasols-style-roast-beef-po-boy-recipe/