I've had a chance to implement the duct split then NewUnionFitting or NewTransitionFitting approach but without success. Both of those methods take two connectors as arguments so there does not appear to be a way to ask them insert a damper. The result of either of those methods is that my split ducts are re-joined.
Looking through the list of methods on Creation.Document I can't see any methods that take two connectors and a FamilySymbol as parameters which I expect I need to be doing. There are a number of other special methods like NewFlexDuct which take two connectors and then a FlexDuctType but I can't find anything similar for duct accessories.
Any ideas on duct accessory insertion between two connectors?