I'm not aware of any clearing firm or broker that will execute a buy-in from a margin call before the markets open. They will either allow you time to liquidate that stock or execute a MOO order to get rid of the call. If you are concerned, you should ask for their procedure in an email so you have it in writing.
Of course they can. If you are holding an option that would cause a large deficit upon expiry, I'd call you in. Other possibilities would be for markets where you haven't agree to hold short positions and a firm must buy you in to avoid going short.