You're on the right track with the Texas Style rub. 2/3 course black pepper 1/3 kosher salt. And thats it.
Now, in Texas we usually cook packer cuts. Lots of fat to take care of the moisture. Flats are usually closely trimmed. So your idea of a roasting pan is spot on.
Let the flat smoke on the grate for about 2 -4 hours depending on the size or you cut at smoker temp of 225. Then put it in your pan with some moisture. Beef broth would work. If you shake a little garlic powder and just a touch of oregano in the broth that would be even better. Cover the pan with foil when the internal temp hit 175 degrees. Use a thermometer to check the internal temp of you meat.
When it hits 200 degrees test your brisket for tenderness. Just stick it with a fork. Feels tender? Take it off. Let it rest for at least 1 hour before you serve it.
Let us know how it turns out!