Available in LEADTOOLS Medical Imaging toolkits.
L_DicomGetOverlayAttributes
L_DicomGetOverlayActivationLayer
L_DicomGetOverlayBitmap
L_DicomGetOverlayBitmapList
L_DicomGetOverlayCount
L_DicomSetOverlayAttributes
L_DicomSetOverlayBitmap
L_DicomSetOverlayBitmapList
L_DicomDeleteOverlay