You are correct, this option to ignore a dimension in sorting while addressing on it is not an option in Tableau currently.
There is an option to work around this. What you can do is create your own INDEX() function with a formula like:
IF LOOKUP(MIN([Product]),-1)=MIN([Product]) THEN PREVIOUS_VALUE(0) ELSE PREVIOUS_VALUE(0)+1 END
(thanks to Richard for this, via http://www.tableausoftware.com/support/forum/topic/top-x-shows-more-x-values-when-using-color-shelf )
Then you can setup your advanced compute using as in the attached image. This works because you are sorting on a text field. If you wanted to sort the products based on each product's total, you might be able to achieve that with the "Show Me" button (see above link for details).
alt_index.twbx 6.5 KB