This issue was brought up in a reddit thread. And now I'm genuinely curious, the person I was talking to about it seems to feel that the major mining players could collude to exclude the attacker, and that would be an acceptable course of action.
So less than 50% of hashing capacity will ignore the longest chain?
There's a slight chance this would work if the attacker's capacity cannot be sustained. For instance, if the attacker is a pool that miners will abandon once they determine that the pool is attacking. It might be possible then for the "honest" side to attract enough hashing to have a decent chance of eventually becoming the longest chain.
But with "longest chain wins", every second that passes makes it more risky for the "honest" miners to even try such a tactic.