Currently, possessing the king is extremely dangerous because you can be caught out by your inability to use Royal Finger.
Therefore, I suggest just giving Royal Finger to any Possessor who takes the king (until they switch bodies again.) This is a buff for them, of course, but possessing the king is extremely risky anyway, since they’re usually guarded and obviously are the center of attention, so I think it’s fine to give them a benefit for doing so successfully.
And needless to say, it also makes thematic / lore sense.