Have you tried just searching for the IAH-Cabo (or IAH-Cancun) flight by itself to see what dates are available? If you can find standard avails, you might be able to call an agent and get them to help you add in the domestic segments; they may be available independently or may be available via a long layover which would not pull up automatically but could be manually built-up.

In many cases, yuo will find things like GRU-IAH is available in your cabin standard reward, IAH-OAK is available in your cabin standard reward, but CO.com will not give you GRU-OAK in that cabin at standard level. Why not? Beats the heck out of me. But FT'er channa alerted me to this and I have now found it to be true 3 times, and all 3 times I was able to get a phone agent to built the record for me. Just research it and know what dates are available and exact flights and get them to input the segments manually as two legs.

Of course, there may really be no avails at all, but this just might help you. Good luck!