How can I get plugin meta data without knowing the plugin folder?

If you know a function or class of the plugin’s main file, you can use the Reflection API.

Example with my plugin T5 Taxonomy Location

add_action( 'plugins_loaded', function()
{
    $class="T5_Taxonomy_Location";

    if ( ! class_exists( $class ) )
        return;

    add_action( 'admin_footer', function() use ( $class )
    {
        $reflection = new ReflectionClass( $class );
        $file       = $reflection->getFileName();
        $data       = get_plugin_data( $file );

        print '<pre>$data=" . esc_html( var_export( $data, TRUE ) ) . "</pre>';
    });
});

Result

$data = array (
  'Name' => 'T5 Taxonomy Location',
  'PluginURI' => 'https://github.com/toscho/t5-taxonomy-location',
  'Version' => '2012.09.11',
  'Description' => 'Creates a new taxonomy for locations. <cite>By <a href="http://toscho.de" title="Visit author homepage">Thomas Scholz </a>.</cite>',
  'Author' => '<a href="http://toscho.de" title="Visit author homepage">Thomas Scholz </a>',
  'AuthorURI' => 'http://toscho.de',
  'TextDomain' => 'plugin_t5_tax_location',
  'DomainPath' => '/languages',
  'Network' => false,
  'Title' => '<a href="https://github.com/toscho/t5-taxonomy-location" title="Visit plugin homepage">T5 Taxonomy Location</a>',
  'AuthorName' => 'Thomas Scholz ',
)

For a function, use ReflectionFunction. It inherits the method getFileName() too.