I think I got it worked out. I went a little different route, but it seems to be working so far.
As I loop through the shopping cart I'm setting a session var for DiscountProductID when the ID of the current cart product matches the ID associated with the discount code. Then I created new eCart discount rules for Product Specific Flat and Product Specific Percentage that are just like my regular discount code rules except that they look for the DiscountProductID session var in order to get triggered. My percentage rule here, though, was based on an items' column value where I used the DiscountProductID session var to pull the line item total for that ID accordingly. I learned here that I needed to use ".$_SESSION['DiscountProductID']." as oppose to just $_SESSION['DiscountProductID'] or it created syntax errors in eCart_PHP.php.
Then I adjusted my original rules so that they included a custom expression of !isset($_SESSION['DiscountProductID']) so they wouldn't get triggered if the code had flagged a product ID.
That seems to have done it! Thanks for your help. I may come back if I find bugs in what I've done. :)