Not something I heard of before but, you could script one like this:
1) Take your "social_identifier_string" of choice
2) make a QR code sized 1 pixel per dot of it.
3) convert the QR image to SVG
4) insert the SVG downsized to 1 pixel into your cover or chapter separator / header / deco... SVG
As long as it isn't known where exactly it is people wouldn't even find it by stringsearch being converted to small dots in svg-lese inside another svg. Eyeball-search of every SVG in the epub under extremely high magnification because every dot of the image could be the QR code sized down.
Having never heard about such attempt of implementing social DRM I assume in good faith that there is no preceding case of it and hereby declare this idea open source
