I had the same question. For ex., Pittsburgh requires 3 science letters from prerequisite classes for individual letters but they said a committee letter from my undergrad in lieu of these letters is acceptable.
I hope this information is accurate because I finished undergrad 7 years ago and most of my teachers are gone, but the school is writing a committee letter for me anyway. I am doing exactly what you are and sending those letters to my school to be included in the committee packet (2 science teachers, 1 boss, 1 dentist). Let us know if you hear of anything else that changes things.