Adding a "Layout Update XML" textarea to a custom module?

I have done that customization for AheadWorks blog once . You can try to adjust this to fit your code.

First add custom_layout_update database field for your entity. I did following to add that for blog post using sql installer:

$installer = $this;
$installer->startSetup();
$installer->run("ALTER TABLE {$this->getTable('blog/blog')}
    ADD `custom_layout_update` text NOT NULL;
");
$installer->endSetup();

Then in blog post edit form I've added field which lets me to save layout update.

Finally I replaced code in renderPage() method which loads layout. I removed this one:

$action->loadLayout();

And added following:

$action->getLayout()->getUpdate()->addHandle('default');

$action->addActionLayoutHandles();
$action->loadLayoutUpdates();

if (strlen($page->getCustomLayoutUpdate())) {
    $action->getLayout()->getUpdate()->addUpdate($page->getCustomLayoutUpdate());
}

$action->generateLayoutXml();
$action->generateLayoutBlocks();

That part inside if condition does the trick, rest around comes from loadLayout() method.

In AheadWorks Blog extension renderPage() method is placed in helper class and it's called in controller. Usually this layout code is being called directly in controller, so you may look in your controller code for that.

I'm not sure you can achieve this using the system->configuration settings.
I mean you can, but you have to load the layout update before any output is generated.
I assume your sliders are used like {{block type=....}} or {{widget type=...}} in cms pages or blocks.
If that is the case, then you cannot load your layout update from system->configuration, because at the time the {{}} directives are parsed the layout handles is already loaded and you cannot add any more handles to it.

If you have separate pages for the sliders and you know in the controller that you are going to use a specific handle you can do the same things as the cms pages do.

$configLayoutUpdate = Mage::getStoreConfig('path/to/update');
$action->loadLayoutUpdates();
if ($configLayoutUpdate) {
    $action->getLayout()->getUpdate()->addUpdate($configLayoutUpdate);
}