I honestly think it's extremely hard to simply go in and be able to get a 30 unless you instantly purchase a CRAZY amount of other items and with being able to find a nice SA. The Madison store does get B's in but lately seem to be pretty backed up, and they seem to only want to offer them to familiar faces/long time clients. The 30 is a extremely popular size and if you do spend that CRAZY amount of money...it would probably take at least a month or two to actually get that phone call when the bag gets into the store.
I would recommend paying a visit to all the Hermes locations in NYC (ie. Wall Street, and in Americana Manhasset in Long Island, and just ask around. You never know and may get lucky! However, the chances are pretty slim.
GOOD LUCK AND ENJOY YOUR TIME IN NYC!!!