Add rotation toggle to powerbutton menu? Is there any way (short of reflashing to a totally new ROM) to add a "toggle auto-rotate" or "toggle portrait/landscape" option to the menu that comes up when you press and hold the power button? 

I'm endlessly fighting with auto-rotation. For whatever reason, every Android device I've owned ends up being over-eager to rotate, but stubbornly resistant to rotating BACK, even when my gestures should be making it completely obvious that I didn't want it to rotate. However, every time I disable auto-rotation, I end up finding myself in a position where I have to rotate to landscape for something, and have to kill even more time trying to re-enable it.

For what it's worth, I have both a Galaxy S3 and a Xoom (wi-fi) (S3 running ICS, Xoom running JB).
In the more recent updates to the Galaxy S III (at least Android 4.1.2) you can modify the notification panel at the top of the notifications screens that you get when you drag from the status bar. Open settings, 'Display' (under 'Device' heading) then in the 'General' heading choose 'Notification panel'.

Here you should be able to choose which Quick setting buttons you want to appear and one of them is the toggle for 'Screen rotation'.