Hamilton Cancellation Line - Sat/Sunday tkts?
I've successfully left a suitcase at the Palms restaurant (right across from the Gershwin, I think it's on 50th) once before. They'll hold it in their coat check area if you ask nicely. There's no fee, but I gave a $5 tip when I picked the bag back up. Some of the theatre district hotels will probably do this too (maybe try the Marriott Marquis, right next to the Richard Rodgers; they are accustomed to dealing with Hamilton cancellation line folks running in and out all day for restrooms/etc,...