In a small saucepan, bring the rhubarb and water to a simmer and cook for 5-6 minutes, until the rhubarb is very soft.
Pour into a bowl and set aside. Add the remaining ingredients and bring to a simmer; cook for 10-15 minutes, until the mixture thickens.
Puree with a hand-held immersion blender or cool and puree in a blender or food processor until smooth. The sauce will keep in the fridge for up to a month—store any surplus in the freezer.
Meanwhile, cover the ribs with foil on a rimmed baking sheet and roast at 325˚F for 2 to 2 1/2 hours, until very tender.
Brush with rhubarb sauce and toss on a hot grill or turn the heat up to 425˚F; roast for another 20 minutes (or grill for 5-10), or until caramelized.