Preheat the oven to 350 degree F; line a baking sheet with foil.
Halve the squash lengthwise and scoop out the seeds. Season the flesh generously with salt and brush with 1/4 cup of the barbecue sauce. Arrange flesh-side down on the prepared baking sheet and roast until tender and the squash strands are easily separated with a fork, 45 minutes to 1 hour. Let cool for a few minutes on the baking sheet.
Meanwhile, whisk together the maple syrup, tomato paste, 2/3 cup of the vinegar, remaining 1/2 cup barbecue sauce, a pinch of salt and 1 cup water in a small saucepan. Bring to a boil, then reduce to a simmer and cook until thickened, 15 to 20 minutes. Keep warm.
Mix together the mayonnaise, cabbage, onion and remaining 2 tablespoons vinegar in a medium bowl. Season with salt.
Use a fork to separate the squash strands (keep them in the skins). Divide 1 1/4 cups of the sauce between the 2 halves and mix until the squash strands are coated. Season with salt.
Slice the buns open about three-quarters of the way. Divide the cucumber slices among the buns. Fill each with a generous amount of the squash and top with some slaw. Serve with extra sauce on the side.
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