Hi, Gerardi’s Dwarf is also a Marcoura and it will survive in zone 6 and seems less prone to dying back, although I lost one a few years ago (zone 6 KY), probably for the reason you stated:
“I suspect it can survive colder “fully dormant” temperatures, but what invariably happens is that at the first hint of spring it awakens from dormancy and then is struck by a freeze, causing dieback or even death.”
I think that is the major issue with Macroura in zone 6, especially Southeast USA with fluctuating winters (it’s been a real doozy of one this year!!!) However in milder winters in zone 6 they should do fine, but sustainability and long term longevity remains a big question with Macroura in fluctuating winter zones with intense cold. Experimental at best. However, there are so many excellent and fully hardy Rubra, Alba, Rubra/Alba crosses, better to just go with those unless you want to experiment.