I haven't seen resort codes, but I noticed on the Universal Orlando website, if you click on the MasterCard logo, there will be additional specials if you use a MasterCard.
The Entertainment club offers 50% off Lowes hotels. I paid $10 for memebership, but I think it usually costs $35. But thats worth it if you stay for 3 days or more.
Also found good discounts on Expedia and the company that advertises on this site.