In situations where a local noble assumes the throne and breaks the PU, the former PU holders gets the Restoration of Union CB to re-establish the PU.What I was thinking was more along the lines of:
On monarch death, a noble from the local nobility rises to the throne, and the country that shares dynasty can use the establish PU casus belli.
Unlike how the game currently handles the definition shares dynasty, which means having the same name, I would have handled it differently in my ideal scenario. On the date of birth, the heir shares dynasty with the RMed country with the highest prestige (Well, ideally you could only RM one country or have a proper dynasty tree but that's another story for another day). This would also make the illegitimate child event quite useful too!
As for your idea, I don't get it. There should be no reason you are limited to a single royal marriage. Even in the proper dynasty building game CK2, you can marry your dynasty off to as many lineages as you want.