First, check to see what Langone's waitlist policy is. Some schools have a strict "don't bug us" policy, while others welcome additional information (within reason). I think Langone is in the latter camp, but you'll want to confirm.
Beyond that, it sounds like you have a pretty good idea of what kept you out. If that's the case, then your communications to the admissions office shouldfocus on that. Put another way, if you think your quant skills kept you out, then it probably won't help to write an additional essay about your fit with the program. Even better than taking a class later (if you don't get in) is to sign up for a class NOW, and to let Stern know that you're taking this class. That shows your commitment to getting in and hepls address this perceived weakness.
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