Funny, many people work on the same thing without knowing ;-)
I did this as well and already created an plugin-page at
http://wiki.splitbrain.org/plugin:showlogin.
I would be interessted in sharing our experience to optimize and maybe merge our plugins into one. It's also not a big thing, but who knows...
There is also a german thread about this topic (opened by me) here:
http://forum.dokuwiki.org/post/6454