Alrighty!
I've got it, was actually easier than I thought:
In /core/codeon.config.php add the following to the top (inside the PHP tags):
session_start();
if(isset($_GET['template']) && $_GET['template'] != ''){
$_SESSION['template'] = $_GET['template'];
define('CURRENT_SKIN',$_GET['template']);
}
if(isset($_SESSION['template']) && $_SESSION['template'] != '')
{
define('CURRENT_SKIN',$_SESSION['template']);
}
Now, the only thing YOU have to do is add a dropdown or links to change the skin. To change the skin, just link it to something like this:
Any page, just append ?template=TEMPLATEFOLDERNAME and it will set the skin.
Note however that if someone tries to become a smartypants, and put in a skin that doesn't exist, they'll get a blank page.