class ArchiverTar implements ArchiverInterface {

  /**
   * The underlying Archive_Tar instance that does the heavy lifting.
   *
   * @var Archive_Tar
   */
  protected $tar;

  public function __construct($file_path) {
    $this->tar = new Archive_Tar($file_path);
  }

  public function add($file_path) {
    $this->tar->add($file_path);

    return $this;
  }

  public function remove($file_path) {
    // @todo Archive_Tar doesn't have a remove operation
    // so we'll have to simulate it somehow, probably by
    // creating a new archive with everything but the removed
    // file.

    return $this;
  }

  public function extract($path, Array $files = array()) {
    if ($files) {
      $this->tar->extractList($files, $path);
    }
    else {
      $this->tar->extract($path);
    }

    return $this;
  }

  public function listContents() {
    $files = array();
    foreach ($this->tar->listContent() as $file_data) {
      $files[] = $file_data['filename'];
    }
    return $files;
  }

  /**
   * Retrieve the tar engine itself.
   *
   * In some cases it may be necessary to directly access the underlying
   * Archive_Tar object for implementation-specific logic. This is for advanced
   * use only as it is not shared by other implementations of ArchiveInterface.
   *
   * @return
   *   The Archive_Tar object used by this object.
   */
  public function getArchive() {
    return $this->tar;
  }
}

hook_archiver_info in modules/system/system.api.php

Declare archivers to the system.

og-7.x-1.x.database.php in sites/all/modules/ulmus/og/tests/og-7.x-1.x.database.php

Filled installation of Drupal 7.0, for test purposes.

system_archiver_info in modules/system/system.module

Implements hook_archiver_info().