Massachusetts would work.How about New England is several states with the dominant one being Massachusetts, with a High Governor who styles himself like the Kennedys and High Kings of Ireland in Boston. Nantucket and Martha's Vineyard are semi-independent baronies grown wealthy on whaling and sea trade with routes to South America and the Old World they keep secret even from the ruling court, causing alot of tension. The old wealthy families died in the Regression and the new aristocracy of sea trade is descended from the large Portuguese fishing population in Mass. Part of the Maine coast is annexed on old historical claims, but Quebec and the locals often fight them for it.
Nantucket and Martha's Vineyard are semi-independent baronies but they're under the control of the rump USA. Consult the East map. Personally doubt that USA would be keeping trade routes to South America secret because it's clear as day: sail south.
I personally think that routes to the Old World aren't all that complicated either. Follow the Gulf Stream, pack enough food and water for a month. Or go to Newfoundland, wait for those Greenlanders to show up, and take their boat back to Greenland, rinse and repeat in Greenland and Iceland and you'll arrive in Norway.
Add in Cape cod, long island and Coastal Connecticut to the rump USA as well.
I think we've speculated about Quebec. I personally think that the Maine Appalachians are too barren for an army to march through, but French speaking catholic raiders is more probable.