King's disposition could still easily be explained and integrated in Assertn's general theory. It just needs someone with a stupidly strong desire and belief in something. Luffy's aim of becoming the pirate king has been shown to be based on rather naive and stubborn belief and little else, seeing how it's basically the shounen plot alone that has kept him alive in many fights against opponents stronger than him. Boa on the other hand has a similar belief in her own beauty and the right to do anything because everybody will forgive someone as beautiful as her. That's hardly different from Luffy's dream.

If such feelings are genuine, it means they will stop at nothing save death on their way to their goal. One can't have a much stronger ambition than that and thus stronger power source for this haki technique.