You were right, of course, but being the db person that I am I had to do it... here is the sql statement if it helps anyone.
update products,products_description,products_to_categories,categories_description,manufacturers set products_description.products_head_keywords_tag = concat(manufacturers.manufacturers_name, ',', manufacturers.manufacturers_name, ',',categories_description.categories_name, ',', products_description.products_name)
where products.products_id = products_description.products_id
and products.products_id = products_to_categories.products_id
and products_to_categories.categories_id = categories_description.categories_id
and products.manufacturers_id = manufacturers.manufacturers_id
[and categories_description.categories_name = "use_this_to_limit_to_single_category"];
I make no guarantees on this, of course..