I did mine exactly as you are describing. I rested a 2x4 board about 10 inches tall on the inside of the cabinet and ran my mounting screws through the wall in the bedroom into the back of the cabinet and 2x4. It seems very solid but I still take the TV down while traveling because it's heavy. I'm sure that others may have a better or more cosmetically pleasing method but I like Redneck engineering. In the bedroom you can only see the mount itself, and the 2x4 can only be seen when you open the cabinet. The wall is hollow so I was careful not to run it down so tight that it would crush the wall.
Bryan, Pam and Cameron
2003 Jayflight 27BHS, 2004 Ford Expedition