If you can't find IAH-DFW (or other US gateway) SAAver seats on your preferred dates, just book from DFW or the gateway with EY seats. You can either change when seats open or buy a cheap IAH-DFW (or gateway) ticket.
In addition, there are some maximum mileage rules that may require separate domestic ticket depending on your origin and destination. Don't know about IAH, but I've always had to buy a separate DEN-IAD ticket to connect to IAD-AUH-XXX.