How to add attributes to menu and menu items on the block level in THEME_preprocess_block()?
There are actually functions available for specific menus too:


  Targeted overrides (suggestions) available for theme_menu_link() and
  theme_menu_tree()
  
  In addition to other changes to menu rendering, a Drupal 7
  theme can implement a THEMENAME_menu_tree__MENU_NAME() and/or
  THEMENAME_menu_link__MENU_NAME() function to override
  theme_menu_tree()/theme_menu_link() for a specific menu. For example,
  THEMENAME_menu_link__management() would override theme_menu_link() for
  links within the "Management" menu. This is similar to how
  "node--article.tpl.php" overrides "node.tpl.php".


Source: https://drupal.org/node/254940#theme-suggestions-for-menus