You can add as many arguments as you want to the register_post_type() $args as they are all save in $wp_post_types global in an array (ex: $wp_post_types[type] = $args), but i’m not sure if that is the best way to go since this could easily be changed in the future and then your functions will break, a better way to go would be to create your own option in the options table and use that, what i mean is that you could use get_post_types() and save them as an array in a type => $args manner and that way you are not depending on the core functionality to stay in the current state. Hope this makes scene.
