You could wrap the flannel around the seam allowance to the front and zig-zag along the edge to hold it in place. As long as you trim the flannel beyond the stitching line for the binding, it won't show on the front. After zig-zagging, you could even fold the flannel back again towards the edge and perhaps machine baste it in place. As long as you keep the width of your zigzag within the seam allowance, the extra flannel inside the binding won't show.