In the original issue Adam Nicols commented:
Hmm- this would be fundamentally difficult to achieve. Help strings are generated by help_ajax.php in the first instance, and a fallback to opening in a new window at help.php. We'd have to somehow pass any filler content in the URL to help.php to make it work in fallback and that's just a big nono really.
Not sure if something changed in the way this works, this comment is from 2013, if time permits at some point I might take a stab at it.