There are a lot of question marks in your comment and I'm sure some of them are meant to be rhetorical and maybe some aren't. Also you mentioned something about exchanges talking to central banks which isn't something that happens. Your fictional example is:
I've got 150,000BTC, please convert that into dollars for me at the market rate. What? No worries I'll just wait here. You can wire the dollars to this account here ...
That isn't the kind of thing you do at the teller window. That kind of liquidity doesn't exist in bitcoin (though it would be a cinch in other major currencies). At this point you are in need of a really clever broker to find you someone to take the other side of the trade at a price that doesn't totally suck. Though that probably isn't very likely...
You seem to imply that the exchange has some responsibility here that I don't understand. The exchange matches buyers and sellers at a price they can both agree on. If there are no buyers for what you are trying to sell, that's too bad but it isn't the exchange's problem. Go look on the CME product list and you'll see tons of contracts that are listed but have no bids because nobody is interested in trading them.
If you want to trade a shitload of a questionable thing then you just might find there's no bid. See also: people selling mortgage backed securities in 2008. The price for not being able to sell a thing is paid by the people that own the thing (especially if they purchased the thing with borrowed monies and they are forced to price the thing by marking it to the market).
I think maybe your question is just "What happens if you want to sell bitcoins and nobody wants to buy them?" and in that case the sellers are just screwed.