Is there any evidence that viable cells are in the menstrual flow?

Yes: Endometriosis is a condition in which endometrial cells (cells from uterine lining) grow in locations other than inside the uterus. We know that women experience "retrograde flow", or flow of menses back thru the fallopian tubes and into the pelvis. Usually, the body resorbs the material, but sometimes these "viable" cells take hold and live in the abdominal cavity.

Yes: Red cells in menstrual blood are viable. Some viable endometrial cells are also present in the menstrual fluid.