Michael, what if to draw the attention of inattentive customers by means of showing some kind of tips? Say, a popup which would remind about the necessity to fill in the coupon code.
Not very long ago the 'Pop-up Anywhere' module was released, and it will be compatible with the 4.4.x branch soon. Haven't you considered this tool? I believe it may be useful.
You may consult with Karina, the module author, in this thread:
http://forum.x-cart.com/showthread.php?t=66512&page=2