Actually peasant loyalty shouldn't be what forces law changes. Peasant *power* should be. Logically speaking, if they're not very loyal, they're probably lacking power and couldn't force a law change. If they have a lot of power, then they'd have a lot of influence and *could*.
This is starting to sound like Vicky laws and POP militancy and consciousness stuff... Well, essentially there are 4 types of pops in CK, but their numbers are abstracted, with only events like "towns and hamlets grow considerably" (increases prosperity level of a province).
More or less

I need to figure Vicky out. It looks like SimIsle with guns. In theory that should be my favorite thing in the world.
I also have to admit the lack of POPS-ish entities in the EU games strikes me as a bit odd. It would need a more CK, abstract way of doing it
Although, another route to take...
Sorry, that's more what I was suggesting. I didn't make that clear. The *event* has a prestige hit, not changing the law in the menu. The thing I was getting at is that if you change the law away from whatever was just selected, it'll scale the MTTH to be really *short*, forcing the law change event and another prestige hit. This should be totally possible given what I've seen so far, assuming... the various triggers work the way I think they do.
Another idea for RP. Since the Grand Mobilization thing doesn't cause an event on you vassals (the reason I thought it did was uh... I misread something!), maybe have RP decrease the MTTH for the mercenary getting event. If you want to get really creative you could probably make a *chain* of events to ensure that the mercenaries being generated match your own culture and give it some flavor text like "You can force your vassals to pledge additional troops to your cause" or something.
I also feel like modding the negative improvements to take how much power the nobility has into account. It seems to me like the knights SHOULD be the law back then, so if the nobility has more than 50% of the power in a given province, it should lower the MTTH for the negative improvement removal affects, and raise the MTTH for their placement events. Maybe have clergy power related to the moneylenders.
Not exactly sure how to mod text, though, as it seems to be contained in some big excel-openable file (A csv? I didn't catch the extension), and while it should be possible to mod it I'm not sure how to actually distribute that. I think BOPACK and whatnot added no text, so...
Is there a modding tutorial anywhere? I'm finding bits and pieces of information on the event scripting forum and by just prodding around, but nothing centralized, and that forum seems a bit dead, anyway.
Also interested in what specifically DV adds, especially in terms of scripting.
--- edit---
Put some ideas I had up on nopaste. Effects should also have various positive and negative trait chances for the ruler I guess.
Also I can't think of any good events that would cause royal prerogative. Maybe something associated with having a high prestige and high enough stats to manage a large personal demesne? Or certain traits? Nobody you controlled would ever try to force it on you.
Maybe something like, if you're energetic, proud, selfish, or zealous, you'll keep getting events to switch to RP if you're past a certain prestige threshhold.
Anyway, ideas.
http://pastebin.com/m376896c0
---edit2---
Just found a PDF discussing new things in DV. If I'm serious about persuing these law change events I might wanna pick that up because some of the new features look like they could lead to fuuuun things.
It's also not exactly clear to me whether or not BOPACK and SMACK and DVIP do anything to address these issues either, so if any of my ideas are redundant...
---edit3---
More thought experiments! Someone mentioned making it easier for RP rulers to revoke titles. While you can't do this with events as far as I know, I think it would be possible to write an event that only happened under RP, where you could get a free claim on any vassal that got disloyal enough. Unless you already have immediate claims on vassals or something.