Suppose that after he had a son, Louis VII demands all his dukes recognize male-preference primogeniture (which means a daughter of a sonless king takes precedent over salic hiers of the sonless king) for the Kingdom and the royal demesne.
Fast forward to the time of the hundred years war. The OTL Plantagenets in practice were willing to not claim the throne as long as their lands in Aquitaine was left alone and Normandy (taken from John) was returned.
Would the Valois take seriously the proclamation made decades ago by a long dead king, which was attended by every French duke (for the time) who agreed to it and are also long dead? Would any of the French nobility (most of whoam are not even decented from the dukes who made the promise) take it seriously? In fact, I don't think such a proclamation by Louis would have any butterflies into the HYW or anything other than a history textbook footnote.