It's not really authentic. As far as I know, it originated with the mother of a woman I used to date in the seventies who lived in Savannah, Georgia. It was I who named it "Mid-Carolinas" and for no good reason either.
I like it better than any of the "authentic" sauces. Matters "gourmet" and "upscale" aside; face it, dijon is a lot more interesting than sunny yellow baseball mustard. As soon as I read "shallots" in your post, it seemed like a natural recommendation. You seem pretty happy about tweaking things, so add or substitute as the mood dictates.
MID-CAROLINAS DIJON MUSTARD BARBECUE SAUCE
Amount: Enough for one shoulder
Difficulty: Easy Peasy. Just mixing.Ingredients
1/2 cup dijon mustard
1/2 cup mayonnaise
1 tbs or more Worcestershire sauce (to taste)
1 tbs or more hot sauce (to taste)
1/4 cup + more tbs (real) maple syrup or honey (to taste)
1 tsp table salt (table salt is better for sauces, but you can use 1-1/2 to 2 tsp kosher salt)
2 tsp (about) freshly ground pepper -- just enough to show up and give the sauce some visual interest
2 cups cider vinegar
1 cup vegetable oil (corn oil is a good choice)Technique
In a bowl, whisk the mustard and mayonnaise to combine.
Add the hot sauce, syrup, salt and pepper, and whisk them in.
Whisk, a little at a time, the vinegar into the mustard/mayonnaise mix. The reason to do this bit by bit is to thin the mayonnaise and not break it.
Whisk the oil in, also a little at a time, or in a slow steady stream, in order to fortify the emulsion.
Add the vinegar a bit a time, whisking in order to prevent the mayonnaise from breaking up.
BDLAnodyne PS. This recipe is original with me. If you like it and want to share it with someone else, you have my permission on satisfaction of each of the following conditions: First, your sharing is not for gain; and second, you attribute the recipe to me, Boar D. Laze. I would consider it a kindness if you would also mention my eventually to be finished book, COOK FOOD GOOD: American Cooking and Technique for Beginners and Intermediates.