AO3 Skins

You'd need to create a new skin, choose the "work skin" option rather than the "site skin" option, and use the "content" tag in CSS. I don't think it would get you all you want for the side column, but it should in theory work for the footer. Something along the lines of the following should do the trick, but I can't get it to work for myself at the moment. I know some improvements to this functionality are planned for the next release, but I'm not sure if it'll fix everything you need.

#workskin #chapters:after { content: "extra text here"; }