They don't need those 18000.
This is the biggest mistake people make.
Those coins where deposited on bitstamp by users who either wish to use them trade or to sell and get the money.
It's highly unlikely that everybody who deposited those 18000 coins will ask for them next thing tomorrow.
I assume a lot of them will just sell their coins (credited to their accounts) and demand back the usd.
So for those bitstamp can simply pay in usd.
Yes, I would not expect it to be instant. BitStamp would need to have a strategy to transition back to equilibrium but in the meantime they would have a fractional reserve thing going on.
So, what would be the best approach to return to equilibrium?
Most probably, when they open, there will be a bank run and BTC is gonna be requested. Doubtful that all the 200k from their cold will be withdrawn, but it is theoretically possible, and then they will have to buy in a hurry, yes. Even if, in a case of reassuring resolution (i.e. they will state that everybody will be paid in full and they will remain fully solvent) that bank run will be very limited, they will still need to buy those coins somewhere -- because they definitely will be audited. As there are no more auctions and the sum is a little bit too large for an OTC market, probably one of chinese big three will have to help here.