This doesn't actually answer my question. Also, his daughters with Constance being married or bethroed to the Plantagenets were at a time when both parties were considering peaceful coexistence, so that's not impossible.
Constance wasn't married to Plantagenet but to Eustache, the son of Etienne de Blois.
As for the daughters of Louis VII, while some were married or engaged to Plantagenets, it was really not a display of modus vivendi, but rather the usual custom to "seal" alliances and treaties (in the case of Henry the Younger and Marguerite, to ensure that places surrounding Normandy would be passed over). That Capetians and Plantagenets didn't stop to marry each other until the XVth is not a sign that they were considering peaceful coexistence, but that they had a large number of succeeding conflicts that were concluded or temporised trough treaties sealed trough marriage.
It's not a rule of thumb, but regular inter-dynastic marriages can be a sign of general tensions.
This my question: Suppose he makes the announcement, and then well now he has a son and that's completely a moot point. Until Phillip V, there is no need to look at the proclamation because... well there is always a son to go to. So now what?
As said above, by several people : it's really hard to consider Louis VII making such an announcement out of blue, out of necessity. It's then really, really implausible to have such announcement being enforced trough a royal act (that, must it be stressed again, are not about institutional changes).
At the very best I could see Louis VII toying with the idea, before settling the matter on a will (as it was customary, if not institutionally biding, at this time). Then the whole thing is scrapped, and forgotten.