This is my simple solution, it will add an checkbox without overriding 'wplink.js'
add_action('admin_print_footer_scripts', function() {
?>
<script type="text/javascript">
/* <![CDATA[ */
(function($) {
$(function() {
$(document).on("wplink-open", function(inputs_wrap) {
$("#link-options").append(
$("<div></div>").addClass("link-nofollow").html(
$("<label></label>").html([
$("<span></span>"),
$("<input></input>").attr({"type": "checkbox", "id": "wp-link-nofollow"}),
"rel=\"nofollow\""
])
)
);
if (wpLink && typeof(wpLink.getAttrs) == "function") {
wpLink.getAttrs = function() {
wpLink.correctURL();
<?php /* [attention] Do not use inputs.url.val() or any input.* */ ?>
return {
href: $.trim( $("#wp-link-url").val() ),
target: $("#wp-link-target").prop("checked") ? "_blank" : null,
rel: $("#wp-link-nofollow").prop("checked") ? "nofollow" : null
};
};
}
});
});
})(jQuery);
/* ]]> */
</script>
<?php
}, 45);