Basic example of how simple it is to add (slightly) less obvious watermarks to EPUB files with pyepub library and stepic

simplewatermark.py

#! /usr/bin/env python

import os
from pyepub import EPUB  # see https://github.com/gabalese/pyepub
from StringIO import StringIO
from PIL import Image
import stepic

epub = EPUB("startepub.epub","a")

epub_images = [x["href"] for x in epub.info["manifest"] if x["mimetype"] == "image/jpeg"]
container_img = os.path.join(epub.root_folder, epub_images[0])

flo = StringIO(epub.read(container_img))

epub._delete(container_img)

img = Image.open(flo)

im2 = stepic.encode(img, 'Hi, this is a plain text string! You can encode me or whatever')

flo_out = StringIO()

im2.save(flo_out, format="jpeg")

epub.writestr(container_img, flo_out.getvalue())
epub.writetodisk("new.epub")

another method, if interested: https://gist.github.com/broderboy/c2cad83a4855e5454bb3