Since the priest is only allowed one spawning under current rules, then yes it would be illegal to put the Mark of the Old Ones on him as well. Nothing states that a second spawning can be added to any character that didn't already have the option to take two spawnings (and subsequently, why there's no rare slotted three spawning saurus warrior blocks). I understand why you'd want to put the mark on it, both in game and fluff wise, but it might have been unbalanced in the playtesting and thus illegal.