Ask each school. I was on a few waitlists, and I had a new letter of rec from my boss (I'm doing research this year) that wasn't available when I initially applied. I'm applying MD/PhD, so a research LOR was particularly helpful, but anyway, I just e-mailed the director (or in your case, e-mail the dean of admissions), and asked if it would be helpful and appropriate to forward this new LOR. I tried this at 2 schools, and both directors encouraged me to send the LOR. And I just got off one of those waitlists (at a program where I knew the waitlist was unranked and where the committee meets regularly to review waitlisted apps... so I'm assuming the LOR helped). It can't hurt to ask. I also sent a letter of intrest to the school where I was accepted.