We know when we have begun a chained assignment when we process a DUP_TOP with non-null on the stack. Push a NODE_CHAINSTORE onto the stack when this happens, and keep it 'floating' on top of the stack for all STORE_X operations until the stack is empty.
To support versions of Python <= 2.5 which use DUP_TOP in more places, I modified ROT_TWO, ROT_THREE and ROT_FOUR to get rid of NODE_CHAINSTORE on the stack if it is present.
