Yes, in my opinion also think that king could have crown helmets and even some gold plated helmets or armor with gold parts to simulate his great wealth and power as king (they should wear the best armor) and different crown styles as you say, but this can also be differentiated from their battle helmets, like for example more helmets from Eastern Europe for Hungarians and Poles different from Western kings such as French, Spanish or English.
About textures only know that i see in other mods as bellum crucis, deus vult and others... Maybe Lifth and Paleologos cand you help better with their graphics knowledge...
Here some pics about kings in battlefield and their crowns or golden helmets.