We at $CONTENT_FARM fell in love with this envsubst-as-a-service paradigm, and built our own with A/B testing. We then discovered we can apply it not only to our content but also to our bash codebase!
Now looking for funding to add evolutionary algorithms, combined with the best ideas from TRAC and m4, to make this a joy to maintain.
I have no idea how CC BY-SA 4.0 works exactly, is the line in the footer sufficient?
Anyway, you should be able to add any of the $variables in the text as GET parameters and it will spit them back out.