html on these devices have no way to lock the screen. It's not possible at this time. It sucks, but that's the way of it. I discovered this when writing my own webapp in code. So C2 won't give you a solution to this.
However, that doesn't mean you might not be able to adjust. I haven't tried this in C2, but this is what you can try to do.
on device orientation change to portrait
rotate your the game layouts so that it's now back to portrait. however this has the catch that players will still see the rotation and the pop back when you change your layout angle.
alternativly which is what I did
on device orientation change to portrait
put up a different screen and pause the app. Then inform the player that please rotate back to landscape.
However, cacoonjs and appmobi may provide locking options. As I haven't done this bridging as of yet I have no idea if it's possible. but I think it would be.