Thanks for posting the link to "A Woman's Balance: Inversions and
Menstruation," by Mary Pullig Schatz, M.D. - I had come across the article before when my yoga teacher said to avoid inversions during menstruation, and I was somewhat skeptical, like many here. But Dr. Schatz' explanation seemed to make sense.
Dr. Schatz has also written a good book "Back Care Basics -- A Doctor's Gentle Yoga Program for Back and Neck Pain Relief" - the book details modifications for many traditional yoga poses to make them safer.