You really need to look for a last minute deal, someone who cancels can't easily sell on as their personal details are in the system now, locked in against their cruise ref plus cruise companies mainly don't allow it. Plus via this website I don't think it is easy not impossible to contact some direct as the only details you have is their name and town/city, so a lot stacked against you but perhaps someone will come up with a solution so keeping checking the responses to your question.
We went on a short mini cruise on Jade in June and based on what we experienced I suggest you choose an alternative ship for your cruise. We certainly could not recomended this ship. Check out the June & July reviews on this site. Consider yourself lucky if you cannot get a cabin.
Reading some of the reviews for this ship, I'd be suprised if your not overwhelmed with offers.