Hi
I was looking for the same problem,and after 2 days I find how to edit meta tag of home page of CMS (not home page and not index.php)
I'm talking about this page:
http://www.url_of_yo..._cms_category=1
-When you are on you Back office:
# select one categorie to edit.
# change number on "bar adresse" to 1
# "xxx/index.php?tab=AdminCatalog&id_category=2" put 1
and you have all meta of home page of CMS in sitemaps
Hope thats help