The Wikipedia Library notification (T132084) currently has no secondary link target when viewed on-wiki. This might be confusing to users who could view it as merely a notice, rather than a link which will take them somewhere. Adding a secondary link target would help with this. Rather than duplicate the destination of the notification overall, we can link to an on-wiki page. This has the other benefit of providing an on-wiki venue for discussion.
The text should be 'The Wikipedia Library', and the icon should be https://commons.wikimedia.org/wiki/File:OOjs_UI_icon_article-ltr.svg.
Original task description
I also note that the URLs end up like https://wikipedialibrary.wmflabs.org/?markasread=4013139&markasreadwiki=metawiki - which I assume those query parameters aren't being respected exactly.
Another option could be to add TWL to the Interwiki map and use links like https://meta.wikimedia.org/wiki/Special:GoToInterwiki/google:foo to provide an interstitial and have the markasread query parameters work.
...and it seems like we're reaching the point where we should consider moving TWL into production. That's probably a separate ticket/discussion though.