Update: Just re-read my synopsis and description:
By "hidden fields" I am actually meaning "hidden questions", sorry for using the wrong term.

Thomas, is that supposed to work?

This works properly in 1.92+. Default values for hidden variables are saved.

In browser responses the hidden field is shown - it's hard to see because that question has no question text. (and so that particular row no header).

TMSWhite: Where in EM is the logic to save the default value for a hidden question? Because I can't find it...

Carsten-

This code in em_manager_helper.php::_ValidateQuestion() sets the default values:

// Regardless of whether relevant or hidden, if there is a default value and $_SESSION[$LEM->sessid][$sgqa] is NULL, then use the default value in $_SESSION, but don't write to database
// Also, set this AFTER testing relevance
$sgqas = explode('|',$LEM->qid2code[$qid]);
foreach ($sgqas as $sgqa)
{
    if (!is_null($LEM->knownVars[$sgqa]['default']) && !isset($_SESSION[$LEM->sessid][$sgqa])) {
        // add support for replacements
        $defaultVal = $LEM->ProcessString($LEM->knownVars[$sgqa]['default'], NULL, NULL, false, 1, 1, false, false, true);
        $_SESSION[$LEM->sessid][$sgqa] = $defaultVal;
    }
}

And this code, in ProcessCurrentResponses() saves the hidden default values to the database:

if ($qinfo['info']['hidden'] && !isset($_POST[$sq]))
{
    $value = (isset($_SESSION['survey_'.$LEM->sessid][$sq]) ? $_SESSION['survey_'.$LEM->sessid][$sq] : '');    // if always hidden, use the default value, if any
}
else
{
    $value = (isset($_POST[$sq]) ? $_POST[$sq] : '');
}

However, there is a bug in that code - it should be $_SESSION[$LEM->sessid], not $SESSION['survey'.$LEM->sessid], which is probably the source of the error.

Fix committed to Yii branch: http://bugs.limesurvey.org/plugin.php?page=Source/view&id=9417

Thank you very much for the hint!