Actually, you can.

Goto:
Gift Certificates/Coupons
Coupons Admin
Select you coupon code (or if you don't have one, create one)
After it returns to the admin page, select restrictions
If it's a specific product, select the category name under product restrictions.
Select the product name from the dropdown.
Select allow from the dropdown.
Select add then back.

This should now discount that specific item when the coupon code is entered.

Tony