tianon

Applying the following patch to wp-includes/class-wp-xmlrpc-server.php solved our problem:

diff --git wp-includes/class-wp-xmlrpc-server.php wp-includes/class-wp-xmlrpc-server.php
index 5e9bbd4..9a24187 100644
--- wp-includes/class-wp-xmlrpc-server.php
+++ wp-includes/class-wp-xmlrpc-server.php
@@ -182,6 +182,7 @@ class wp_xmlrpc_server extends IXR_Server {
                        return false;
                }

+               do_action_ref_array('wp_authenticate', array(&$username, &$password));
                $user = wp_authenticate($username, $password);

                if (is_wp_error($user)) {

Just needed to process “login” in the same way as user.php to allow Simple LDAP Authentication’s wp_authenticate plugin to do its dirty work.