I deleted my last comment so as not to confuse the issue here.
Having read read MDL-40160 it should be possible ti create a custom login page if you follow the instructions found in...
../admin/settings.php?section=manageauths
By creating a customised login page where the action in the login form points to your own site's ..login/index.php
I seem to recall Shaun Daubney having a similar problem in the older Aardvark themes, he got round it eventually using something like this...
Copyright 2011 Shaun Daubney (Aardvark theme)
<?php
function get_content () {
global $USER, $CFG, $SESSION, $COURSE;
$wwwroot = '';
$signup = '';}
if (empty($CFG->loginhttps)) {
$wwwroot = $CFG->wwwroot;
} else {
$wwwroot = str_replace("http://", "https://", $CFG->wwwroot);
}
if (!isloggedin() or isguestuser()) {
echo '<div class="profilelogin" id="profilelogin">';
echo '<form id="login" method="post" action="'.$wwwroot.'/login/index.php?authldap_skipntlmsso=1">';
echo '<ul>';
echo '<li><input class="loginform" type="text" name="username" id="login_username" value="" placeholder="'.get_string('username').'" /></li>';
echo '<li><input class="loginform" type="password" name="password" id="login_password" value="" placeholder="'.get_string('password').'" /></li>';
echo '<li><input type="submit" value=" '.get_string('login').' " /></li>';
echo '</ul>';
echo '</form>';
echo '</div>';
echo '</div>';
} else {
echo '<div class="profilename" id="profilename">';
echo '<a href="'.$CFG->wwwroot.'/user/view.php?id='.$USER->id.'&course='.$COURSE->id.'">'.$USER->firstname.' '.$USER->lastname.'</a>';
?>
Cheers
Mary