With the advent of the iOS 6+ Accessibility feature "Guided Access", there is a workaround for this. It limits a few things but it completely works if you really need a solution.
To enable the feature you will use, navigate to Settings > General > Accessibility > Guided Access and flip the toggle to ON. Set a passcode.
Navigate to the app you wish to lock rotation in, and rotate the device to a landscape orientation.
Triple-click the Home button. Depending on how other Accessibility options are set, you may need to then select Guided Access from a popup that appears.
Select Options and disable Motion. Press Done, followed by Start.
This will now lock the rotation in a lanscape orientation! Unfortunately it also disables all hardware buttons, so you will be unable to switch apps, lock the device, or adjust the volume until you triple-click the Home button again to disable Guided Access.