I would definitely not assume that rush is readily available.
A friend of mine tried to get tickets for us on Saturday, November 21 and by 10:45am they had sold out of rush for the evening performance. Perhaps they release more rush seats during the day depending on the day-of sales, but he bought tickets to another show immediately after, so we didn't check back.
It sounds like some have been successful with rush right before curtain, but the last week or so had an odd holiday schedule, so perhaps the Friday matinee wasn't as sold out as a traditional performance because it was at an unconventional time and date?