Changes have been made in Git (master):

commit df11ff4cfa438b417435844b21c55babf3122fb7
Author: Jan Schneider <jan@horde.org>
Date:   Tue Jun 11 16:34:32 2013 +0200

     [jan] Don't overwrite parameters for composite sub-drivers (Bug #12284).

  passwd/docs/CHANGES           |    1 +
  passwd/lib/Factory/Driver.php |    2 +-
  passwd/package.xml            |    2 ++
  3 files changed, 4 insertions(+), 1 deletions(-)

http://git.horde.org/horde-git/-/commit/df11ff4cfa438b417435844b21c55babf3122fb7

The Expect driver works well by itself, with the following backend.local.php

  $backends['expect']['disabled'] = false;
  $backends['expect']['params']['script'] = PASSWD_BASE . 
'/scripts/passwd-expect';
  $backends['expect']['params']['params'] = '-host servername';

The successful command called by Driver/Expect.php is

LANG=C LC_ALL=C /usr/bin/expect -f 
'/horde/passwd/lib/../scripts/passwd-expect' -- -host servername -log 
'/var/tmp/passwdZ4aqha'

Now trying to use Expect with the Composite driver:

$backends['combo'] = array(
     'disabled' => false,
     'name' => 'Passwords',
     'driver' => 'Composite',
     'policy' => array(
         'minLength' => 6,
         'minNumeric' => 1,
     ),
     'params' => array('drivers' => array(
         'expect' => array(
             'name' => 'Unix Server',
             'driver' => 'Expect',
             'required' => true,
             'params' => array(
                 'program' => '/usr/bin/expect',
                 'script' => PASSWD_BASE . '/scripts/passwd-expect',
                 'params' => '-host servername',
             ),
         ),
         'samba' => array(
             'name' => 'Samba Server',
             'driver' => 'Smbpasswd',
             'params' => array(
                 'program' => '/usr/bin/smbpasswd',
                 'host' => 'sambaserver',
             ),
         ),
     )),
);

Expect fail to connect to server.  Debugging the code in 
Driver/Expect.php, the command that it is trying to execute is

LANG=C LC_ALL=C /usr/bin/expect -f 
'/horde/passwd/lib/../scripts/passwd-expect' -- Array -log 
'/var/tmp/passwdiBa4pa'

Note "Array" in the command instead of the script params "-host 
servername"  ($this->_params['params'] in the Expect.php code)

The following is a workaround for the Expect driver, but the bug might 
be somewhere in the Composite driver:

--- Driver/Expect.php.orig      Wed May 29 12:07:22 2013
+++ Driver/Expect.php   Wed May 29 14:16:33 2013
@@ -34,10 +34,12 @@
          // Temporary logfile for error messages.
          $log = Horde::getTempFile('passwd');

+        $params = is_array($this->_params['params']) ? 
$this->_params['params']['params'] : $this->_params['params'];
+
          // Open expect script for writing.
          $prog = 'LANG=C LC_ALL=C ' . $this->_params['program'] .
              ' -f ' . escapeshellarg($this->_params['script']) .
-            ' -- ' . $this->_params['params'] . ' -log ' . 
escapeshellarg($log);
+            ' -- ' . $params . ' -log ' . escapeshellarg($log);

          $exp = @popen($prog, 'w');
          @fwrite($exp, $user . "\n");