Solution to doing some processing for every method of FuelPHP REST controller.

gistfile1.php

<?php

class ExampleRestController extends Controller_Rest {

	/**
	 * Make sure the router method has the same method signature as the parent::router declaration.
	 * @see fuel/core/classes/controller/rest.php
	 */
	public function router($method, array $params) {

			// Run a method that validates the provided API key.
			if (!static::validateMyAPIKey())
			{
				// If it has not passed validation return a FuelPHP Response object.
				$response = Response::forge(Format::forge(array('Error' => 'Failed to authenticate you'))->to_json(), 401)->set_header('Content-Type', 'application.json');
				return $response;
			}

			// The API key is valid, Route the request!
			parent::router($method, $params);

		}

	}

}