Just committed a quick update to my Link Footnotes WordPress plugin. The only addition was a new setting that allows you to set the target attribute for the footnote links created.
There are 3 options.
If you want footnotes on by default on all of your posts, check the box. If you want them off by default, then clear the checkbox.
If there aren't any external links the footnotes won't show, so you likely can leave this checked and not worry about an empty list on posts that don't have external links.
HTML inserted before displaying the links.
New option that allows you to insert a target attribute into the links that are created. Defaults to nothing which leaves out the target. Anything else will insert the attribute. Most likely use would be to use _blank to have the links open in a new window.
And there are also two custom fields that you can put on an individual post to control whether the footnotes show.
lf_on - If you have Default On turned off, setting this meta field will turn on the footnotes for an individual post.
lf_off - If Default On is set, this will turn off footnotes for an individual post.
Both meta fields can have any value, as long as there's a value. lf_off will take priority over lf_on if both are set.
You can get the updated plugin from the WordPress plugin library or through the WordPress admin.