Or, your attacker can pay large portions of previously active stake to go quiet, then wait some amount of time and launch his attack when he knows your selection rule will favour his fork.
How does that work? Bribling?
Yes.
How long does the quiet periods usually last? If I find out that the chain I accepted have some proportion of stakes suddenly disapear for two days, I'll check it again. Late, but still objective.
More importantly, in PoAS, miners are working fulltime with accumulated stakes. If you don't want the real miners suddenly stop working , which is too obvious, you have to buy the accounts of those working miners. I can't figure out how to purchase that yet.
It cannot work. Whatever rule you come up with related to inactive stake, you can easily replicate either through bribing, or just might happen by accident.
In addition, the attackers could easily be miners as well.
You cannot cobble together a chain selection rule + some other bizarre ad-hoc rule for detecting history attacks; they have to be one and the same rule, otherwise this will just be to easy to exploit due to complexity.