To put the question another way: what do you do for a rear derailleur if you want to run a full SRAM drivetrain & shifters on a cyclocross bike having a standard or compact road double crank? Where the rider in question most definitely wants to run a rear cassette of at least 30 max teeth?
You can get the SRAM XX mountain bike derailleur. It will take the 30 tooth you are thinking about. I believe it will take up to a 36. I am sure someone will corect me if I am mistaken.