If the global of the script page function utilizes $userob on that page you can:

Detect a Guest:
Code
	if(!$userob->is_logged_in) {
		echo("Guest");
	}

Detect a Logged in User:
Code
	if($userob->is_logged_in) {
		echo("HAI!");
	}

You could use those bits to set a variable and have a template trigger read the variable and display an island.

If you mean in the sidebars, you'd have to rejigger how they function in order to inject an additional island only for regged users.

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