add-rel-lightbox, the code discussed in this post is now available in the WordPress Plugin Directory. This means that you can install it in your WordPress site by searching for "add-rel-lightbox" in the
Plugins -> Add New page of your site administration.
The code is also hosted at github (github.com/DefProc/add-rel-lightbox/) for you to pull, fork, alter or adjust (git is my preferred revision controller): improvements are accepted.
To make it work, you will also have to have a lightbox/slimbox/lightbox clone installed on your site — either as a plugin, or as part of your theme, however if one is missing or disabled, add-rel-lightbox will fail gracefully and silently. I'm using "slimbox plugin" (also on the WP Plugin Database), which despite not being listed as compatible, continues to work on my up-to-date WP versions.
That is all.