What about a reverse rebellion? Basically scavenge the rebellion code, except the Ruler starts it and is treated as the "rebelling" faction. If the Ruler's faction succeeds in holding control of the capital, then he gets to pick a new government, and if not, he gets booted and banned just like any other rebel leader.
You'll either have to slaughter all the militia in the capital, or convince the Margrave of the capital to get rid of all the militia even leaving aside any elements that might oppose you (whom the game would make clear would be the legally correct people). And either way, if you can pull that off I think you deserve a new form of government.
