I have the DDC and I can still use coupons.
For example, I just used "CJC1COMUK" for a .99 .Com registration
Also, if a coupon that you know works is not going through, try dropping the domain from the cart and add it again and the coupon will be carried over in the URL
$1.49 .com coupon code: cjcrmn149s
$4 for domain with privacy: cjcrmn3cp
I have registered for DDC too and all coupons are working as usual for me unless they are regionally restricted or the discount is lesser than what DDC gives.
Sometimes some coupons don't work when I select to pay in my local currency. If that's what happening with you too, then select USD as the currency in your shopping cart before making the payment.
I'm looking for any domain renewal codes for .org and .com as well. Man this is getting expensive. Next time I get a domain with a promo code - I am definitely getting it for 10 years no less....!