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.