How would you implement function rich forms to your module
I've found the solution today myself. I've been developing Webform components. I've found this way to be the best solution without altering any database structures and still getting the full functionality I wanted to get. 

Instantiating - The base of your webform component

hook_webform_component_info - will be your friend in defining your components base structure. Since I needed the planning which would consist out of more timespaces (could've done better here on the termonology ;) ), I added a component named "planning" and gave it a "feature": group. This way I can extend the component "planning" with the "timespace" component.

Values - Add a limit of subscriptions and other parameters for your timespace

_webform_edit_component - Makes it possible to alter the edit form of your component. Make sure you at least add an "limit_subscription" (adjust naming to your liking) field so we can check against it. 

_webform_defaults_component - Will be needed to inject the defaults to your edit form, which you have created with the function here above.

Cosmetics and security - Make sure you get the checkbox and disable it when full

_webform_render_component - Arranges all the cosmetics on your webform component. So make sure you edit your timespace component to get the checkbox you want so people can check a timespace. Also, you can (ofcourse) add some additional markup to your liking.

Security, checking the limit

This is also the right place to check for your limits and disabling the checkbox when full. (see the function webform_events_check_timespace_limit($component), use that to set #disabled on your element. Also make sure you give your element the right validation, something like this, so they can't screw around by simply enabling the checkbox:

'#element_validate' =&gt; array('_webform_validate_timespace'),


Then check against the database in your validation function for the limits, this is how I got it done:

/**
 * Validation for timespace elements
 */
function _webform_validate_timespace ($element, &amp;$form_state) {
    // If checked, make sure this timespace has some space left.
    if ($element['#value'] === 1) {
        $remaining_subscriptions = webform_events_check_timespace_limit($element['#webform_component']);
        if ($remaining_subscriptions === -1) form_error($element, t('The timespace that you\'ve chosen is already full!'));
    }
}

/** 
 * Check if the limit has been reached, returns remaining count to the limit, or -1 when full, or FALSE when no limit is set
 */
function webform_events_check_timespace_limit ($component) {
    $limit_subscriptions = $component['extra']['timespace_properties']['limit_subscriptions'];
    if (!empty($limit_subscriptions)) {
        $count_subscriptions = webform_events_get_timespace_count($component['nid'], $component['cid']);
        if ($count_subscriptions &gt;= $limit_subscriptions) return -1;
        else return $limit_subscriptions - $count_subscriptions;
    }
    return FALSE;
}

/**
 * Get the count of the current timespace subscriptions
 */
function webform_events_get_timespace_count ($nid, $cid) {
    $select = db_query('SELECT count(*) FROM {webform_submitted_data} WHERE nid = :nid AND cid = :cid AND data = 1', array(':cid' =&gt; $cid, ':nid' =&gt; $nid));
    return $select-&gt;fetchColumn();
}


Backoffice - Exporting, table views, analysis..

For this you need the following functions, I'm not writing out these, I could go publish a book if I would:

Exporting (CSV, Excel):


_webform_csv_data_component - The actual data per row.
_webform_csv_headers_component - The headers in the top of the sheet.


Submission view (The page you get at individual submissions)


_webform_display_component


Table view


_webform_table_component


Analysis view


_webform_analysis_component


More help

Since I've just outlined the basics of this thing, I'd recommend you check out some good examples. 

I've managed to figure this all out and fix get somewhat of the following (of course; early beta screenshots)

Submission view


Analysis view


A simple export


Front-end example


Table view


Setting up a event