I think Authority would be a good attribute to base these sorts of things on. Not sure what it does in vanilla M2TW (according to some other threads, it only affects the likeliness of rebellions & possibly brigand spawning rates), but it's in the game already, so it might as well be taken advantage of.

It could probably be modded such that Generals are more likely to develop rebellious traits (reduced loyalty, lower bribery costs, increased corruption, for example) when the Faction Leader has low authority. And the opposite could be true for having above average authority.

A good question is what exactly should promote or demote a ruler's authority? Haven't played M2TW in a while, so my memory is fuzzy, but I'm pretty sure that your ruler basically gains a point of authority every time a province is captured. This may be hardcoded, but if not, I think it makes more sense to increase rulers' authority when they personally capture cities and/or win battles. I think it's safe to say a king or emperor who involves himself in battles will be more respected and feared than one who does not.