How to change the wp-login.php page title?

It looks like it’s not easily accessible as it’s displayed as ^src:

<title><?php echo get_bloginfo( 'name', 'display' ) . $separator . $title; ?></title>

where the separator is:

    $separator = is_rtl() ? ' › ' : ' ‹ ';

and the $title part comes from:

login_header( $title="Some title" , ... );

But it looks like you’ve already checked this out, as I see you’ve filed a ticket #40812 for an extra filter to change the separator.

A workaround that comes to mind, to change the separator, would be to use output buffering hacks to replace it.