I don't know if this has been done before, but you could choose your template with a _GET parameter.
Somewhere in your conf/local.protected.php, add
$allowed_tpls = array( '', 'default', 'monobook' ); // maybe there is a DW function to detect available ones
if ( in_array( $_GET['tpl'], $allowed_tpls )) $conf['template'] = $_GET['tpl'];
Now appending &tpl=a_template will select it.