simpleSessionSet($key, array());

I needed the same thing (looks like I’m the most active user of this plugin.. :))

I modified the Get and Set functions like so:

function simpleSessionGet($key, $default='') {
    if(isset($_SESSION[$key])) {
        return unserialize($_SESSION[$key]);
    } else {
        return $default;
    }
}

function simpleSessionSet($key, $value) {
    $_SESSION[$key] = serialize($value);
}

The trick is to save all data serialized in the session, and unserialize it again when you need to retrieve it.

If you don’t like to modify the plugin code (even though I believe the code is more robust with my modifications), you could just call the functions like this: simpleSessionSet($key, serialize($value)) and unserialize(simpleSessionGet($key))

In you particular case
$_SESSION['hashed_urls'][$_url]=$value;
would be equivalent to
simpleSessionSet('hashed_urls', array($_url=>$value));
if you used the modified plugin code.

This is such an easy change to make, I’m not sure why you haven’t just pushed it.