well, there is a way, but it would mean cheating:
you see, when you try to persuade a lord, the game takes into account several factors (such as your relation with him, his relation to his current liege, his position in his liege's court, the safety you can offer him militarily etc...). depending on those factors, two numbers are calculated, one for you and one for the current liege and depending on the difference between those numbers (big positive difference is good for you) a chance is calculated (something between 0% and 100%). then the game generates a random number between 0 and 100. if the chance is higher than this number, your persuasion is succesful, if not you have to wait some time until the lord wants to speak about politics again. so in the end, the decision of the lord is a question of probability calculations.
if u choose "toggle cheat menu on" in the camp menu, u get the option "reset lords decision ...blabla" when talking to the lord, letting you try again and again to persuade him until the game generates a radnom number smaller than the calculated chance.
of course u can do it without cheats, too, just by asking him every now and then, but that takes long long time.
i'm afraid i dont know of any other possibilities