Can you become a regent for a king as a vassal?

I was recently thinking of trying a campaign where I'd try to become king via a regency, (getting grey eminence, taking my lieges titles like an ambitious AI might try to do.) but I don't even know if it's possible, my experience as a vassal has only ever had me becoming a marshal and a commander. So is it even possible to become the designated regent of a kingdom or Empire? If so how do I go about getting appointed?