I agree - the guy isn't valuable just because he's an artist, it's because he's a brand champion first and foremost. If you got a lazy artist, they'd love the idea at first and then they'd prioritise their art stuff even without realising and without giving notice ahead of time. Brand champions won't lose interest like that.

Pick people who have irregular work (part-timers, social workers, freelancers) but who are also brand champions.