Good thing I only added the screws/bolts a few weeks back, and they're of the A4 (AKA marine / acid-proof) variety which is supposed to be "more stainless than stainless", if that makes any sense. Just checked my whites and they're unaffected so far. I won't be leaving them in, though. Unfortunately they're not magnetic so getting them out will be a hassle.
I'm aware this is nothing to do with the original topic anymore, but as the OP's question has been answered and the discussion on paint agitators is helpful in and of itself, I trust I'm not doing any harm.