A mod for this, little change to make it work for checkout page:
www.opencart.com/index.php?route=extension/extension/info&extension_id=10076

When Registering New Customer It will creates new random digit number for coupon then Sends to the customer mail id. We can set the validity for this. By default, it's 5% OFF.
This works on account registering page and checkout page.
you can change the amount inside xml to fit your needs.
such as free shipping from 0 to 1 to enable.
total amount, use times,coupon type and amount,etc.

Only tested on oc1.5.5.1.
no support is guaranteed since i'm a user too