Magento OAuth Authentication can not handle custom URL scheme

Your url is not valid in Zend_Uri::check($url) because of 'myapp'. Here is check() function:

public static function check($uri)
{
    try {
        $uri = self::factory($uri);
    } catch (Exception $e) {
        return false;
    }

    return $uri->valid();
}

Let's look how the factory work:

public static function factory($uri = 'http', $className = null)
    {
        // Separate the scheme from the scheme-specific parts
        $uri            = explode(':', $uri, 2);
        $scheme         = strtolower($uri[0]);
        $schemeSpecific = isset($uri[1]) === true ? $uri[1] : '';

        if (strlen($scheme) === 0) {
            #require_once 'Zend/Uri/Exception.php';
            throw new Zend_Uri_Exception('An empty string was supplied for the scheme');
        }

        // Security check: $scheme is used to load a class file, so only alphanumerics are allowed.
        if (ctype_alnum($scheme) === false) {
            #require_once 'Zend/Uri/Exception.php';
            throw new Zend_Uri_Exception('Illegal scheme supplied, only alphanumeric characters are permitted');
        }

        if ($className === null) {
            /**
             * Create a new Zend_Uri object for the $uri. If a subclass of Zend_Uri exists for the
             * scheme, return an instance of that class. Otherwise, a Zend_Uri_Exception is thrown.
             */
            switch ($scheme) {
                case 'http':
                    // Break intentionally omitted
                case 'https':
                    $className = 'Zend_Uri_Http';
                    break;

                case 'mailto':
                    // TODO
                default:
                    #require_once 'Zend/Uri/Exception.php';
                    throw new Zend_Uri_Exception("Scheme \"$scheme\" is not supported");
                    break;
            }
        }

        #require_once 'Zend/Loader.php';
        try {
            Zend_Loader::loadClass($className);
        } catch (Exception $e) {
            #require_once 'Zend/Uri/Exception.php';
            throw new Zend_Uri_Exception("\"$className\" not found");
        }

        $schemeHandler = new $className($scheme, $schemeSpecific);

        if (! $schemeHandler instanceof Zend_Uri) {
            #require_once 'Zend/Uri/Exception.php';
            throw new Zend_Uri_Exception("\"$className\" is not an instance of Zend_Uri");
        }

        return $schemeHandler;
    }

It use Zend_Uri_Http class for http:// and https:// schemes. You just need to add own scheme in list.

How to fix?

1. Copy lib/Zend/Uri.php file to app/code/local/Zend/Uri.php
2. In factory function found 'switch' code block (~line 119) and replace it with next: switch ($scheme) { case 'http': // Break intentionally omitted case 'https': $className = 'Zend_Uri_Http'; break; case 'myapp': $className = 'Zend_Uri_Http'; break; case 'mailto': // TODO default: #require_once 'Zend/Uri/Exception.php'; throw new Zend_Uri_Exception("Scheme \"$scheme\" is not supported"); break; }
3. Save file and clear Magento cache. Now your myapp:// scheme will work as http:// and https://