Interesting - I like it. When you say it "helps a bit", what makes you feel that way? If I'm going to change my cart, it needs to be something simple like what you've done.
(I haven't A/B tested this, so I can only rely on intuition and feedback from other people.) I think that the honesty plays a role here. If there's an empty space that says "put a coupon there", people will be looking for a way to fill it, even if they don't have a coupon -- because this box invites them to. However, if there's a checkbox saying "click here if you have a coupon code" (although my checkbox says something different), it means that by clicking it people will lie, and not everyone wants to do it.
Now that I think of it, maybe even better strategy would be to buy a domain "blabla-coupon-code.com" and put there a coupon code for your thing with a small discount. People who like to search the web for coupons will be satisfied ;)
