New Post: Product on sales when products don't belong to a category

Afaik u cant apart from creating a cart discount, but why not just add the product to a category?
If u dont want the category to display, just hide it by ticking the "Special hidden category" under "Extra" settings in category management.

If u got more products that are sharing the same discount, simply add them to the same hidden category and set a discount for that. :-)