My PI is offering to write a letter in my behalf to the Admissions Committee, basically an additional LOR (he never wrote one for my app because I had just been in his lab for less than a month when I submitted it). He has no ties to Pitt, but is insisting on writing a good letter for me based on my performance in the lab since June (involvement in several projects and collabs, + 1 pub and 1 poster, detailed in a previous update letter). My question is, is this something people on the waitlist can do, or should we just stick to update letters/LOIs?