It depends which Sack we're talking about (there's a couple of different quests that have a sacking mechanic), but if you mean Conflict at the Carrock.. then yes, it looks like you can ready your hero. You likely won't be able to do anything with him/her once they're readied, but the "Sacked!" card doesn't prohibit readying.
However, if you're talking about The Hobbit: OHaUH quest with sacking, then no. Those sacks specifically say you can't ready the attached character.
Sorry, I was talking about the Hobbit sacks.
mmm so the worst case is Bilbo being sacked while he is exhausted because he won't be able to remove any sacks from that moment, even with Unexpected Courage, isn't it?