Digital keys are already used in SN's lightning login system, so there's little need to add another certificate so far as I can tell, it's really up to the admin to decide how to handle squatters at this stage, but I would be inclined to let the free market deal with it. Either the "celebrity" can buy it from the squatter or they can simply use a different but similar nym (like ReallyRealCelebrityMonster44).

It's possible that low-reputation accounts could be purged by an algorithm after a period of inactivity which would eventually reduce squatting.