The only thing I can think of is to limit the system to certain titles that the Pope is more likely to have a say over (i.e. the Sicilies or the KoJ, not Norway or Georgia). Then make those titles catholic-only (perhaps with alternate non-catholic versions) and add an allow condition requiring a modifier papal_approval. Then you'd have a maintenance event that uses variables to count the number of provinces inside the de jure kingdom and compares that number to the number owned. For example, every de jure province would add 1 to the global variable koj_de_jure_size, every owned/vassal province a character owns inside the KoJ would add 1 to his koj_owned_size variable. If a character's koj_owned_size variable reaches whatever number is 50% of the de jure kingdom, it would unlock a decision to seek papal blessing, starting an event chain that would eventually add the papal_approval modifier.
That would be a ton of code, since you'd need a to test each province in the de jure realm every time the maintenance event fires and compare every possible value of de_jure_size to owned_size (well, not every possible value, but a lot in case de jure assimilation shrunk or expanded the kingdom).
