well, this I wanted for a long time as well. Not only for PE, but for all apps.
tough I hate iOS7 for a lot of reasons, this is a good point: it has a screen rotation lock built in in the system in easily avaible anytime. So this would probably be the way to go, and not to include it in PE?
(But be patient: I would not recommend to go to iOS7 before you must, and be warned: no way of going back!!!)
Well after all this time, I learned that iOS 5 has such a feature built in and it’s super easy to access; double-click the home button, swipe left-right once to reveal the iPod controls and there’s a rotation lock icon staring you right in the face. You even get a nice little icon in the top status bar as a reminder it’s active.
Since iOS7.1 (i think) and at least on the iPad its possible to configure the lock switch to either mute sound or lock the screen orientation. I set it to the 2nd as it suits me more. Interesting to note it is possible to build this function into apps as GoodReader has had this function for some time.