Combine rhubarb, sweet onion, soda, apple cider vinegar, brown sugar, molasses, Worcestershire sauce, cinnamon, allspice, salt, black pepper, cloves, ground chipotle, garlic powder, and liquid smoke in a saucepan; bring to a boil, then reduce heat to low. Simmer until rhubarb and onion are very soft, about 45 minutes, stirring often.

Pour the sauce into a blender, filling the pitcher no more than halfway full. Hold down the lid of the blender with a folded kitchen towel; pulse a few times, then puree until smooth. Puree in batches if necessary.