Very much echo CWS sagacity, claim after the event and quite likely it'll be paid. I've done very similar things myself, one being a day trip to Bordeaux that ended up having to be an overnight because of a cancellation. Obviously don't push the boat out to the point of possible financial capsize, you may not get the expenses. I view these things as an opportunity to spend a little more time at the destination with the possibility of having a free hotel night if it works out.