You made me curious so I did a search on FREIDA for prelim OB/GYN programs -- it doesn't obviate the need to get a definitive answer as far as satisfying the anesthesiology requirement, but the programs are out there.
I believe ABA's program information booklet lays out the requirements for what is expected in a prelim year. So even if you had the ABA's blessing, you would have to make sure your program could accommodate those requirements.
Good luck.
Obstetrics and Gynecology Arizona
Phoenix Integrated Residency Program
Phoenix, Arizona 220-03-21-328
California
UCLA Medical Center Program
Los Angeles, California 220-05-31-038
Hawaii
University of Hawaii Program
Honolulu, Hawaii 220-14-31-081
Massachusetts
Tufts Medical Center Program
Boston, Massachusetts 220-24-21-128
New Jersey
Newark Beth Israel Medical Center Program
Newark, New Jersey 220-33-21-321
New York
Bronx-Lebanon Hospital Center Program
Bronx, New York 220-35-11-180
Albert Einstein College of Medicine Program
Bronx, New York 220-35-21-178
New York Methodist Hospital Program
Brooklyn, New York 220-35-31-339
Nassau University Medical Center Program
East Meadow, New York 220-35-31-174
Flushing Hospital Medical Center Program
Flushing, New York 220-35-11-184
Jamaica Hospital Medical Center Program
Jamaica, New York 220-35-21-186
NSLIJHS-North Shore University Hospital/NYU School of Medicine Program
Manhasset, New York 220-35-31-175
NSLIJHS-Albert Einstein College of Medicine at Long Island Jewish Medical Center Program
New Hyde Park, New York 220-35-21-190
Mount Sinai School of Medicine Program
New York, New York 220-35-21-196
New York Presbyterian Hospital (Cornell Campus) Program
New York, New York 220-35-21-197
New York Medical College at Westchester Medical Center Program
New York, New York 220-35-21-199
New York University School of Medicine Program
New York, New York 220-35-21-200
Pennsylvania
Temple University Hospital Program
Philadelphia, Pennsylvania 220-41-21-254