We already did this during the block 202612 attack. We asked them to halt deposits and withdrawals (but not trading!) until we figured out what was happening. The minute the fork resolved and we had a handle on what to do about it we told them they could resume.

If, during that fork, an exchange had continued accepting deposits and processing withdrawals we wouldn't have stopped them, that's entirely at their own risk.

Monero still has a lot of work to do, and we don't have $20 million to rest on, so I can imagine that a sophisticated attack like this might be repeated in future. As it stands right now, a resourceful attacked could probably just fork the network based on hashing power alone. Obviously we will do our utmost to ensure that this isn't possible, but if it does happen we will - again - handle things as best we see fit. We won't ever ask exchanges to pause deposits / withdrawals unless we don't know what's going on and fear that we may be under attack.