I haven't done the rush, but I would guess that it isn't too difficult to get a ticket. The show isn't selling very well, and as this thread demonstrates, there are some who don't even know the rush exists. I might be wrong, but I'd wager that you'll have no problem getting a ticket, especially if you arrive a little early just to be safe (like 9am or even 9:30).
This is just an estimate based on their ticket sales and my years of rushing shows. If anyone HAS done the rush who has more accurate info, feel free to jump in.