Sips image resize: Background cover

Using SIPS, I can proportionally resize an image using sips -Z 256 to reduce the width and height of my image to a size no larger than 256px. Over a padded background fill, this produces a result similar to a CSS background-size: contain resize. However, what I'd like to get is a crop of 256x256, similar to a CSS background-size: cover resize, where the width and height are proportionally reduced to a canvas no smaller than 256px. sips -Z 256 "$file" --cropToHeightWidth 256 256 is close to what I'm after, but the Z flag is leaving one side too small. How would I achieve this result, instead? It would be nice if I could apply something like cv2 to get a background:cover style proportional scale with a crop position which defaults to a center-crop, but adjusts the position to prevent faces from being cropped out of the image.