I have been using the flashBums Bitmapscroller on some projects, as in http://gotoandlearn.com/play.php?id=143. This works for any amount of images you'd like to scroll on your iPad.
Since I really like the elegance of your BlitMask solution, I have used that instead of the Bitmapscroller in my latest project. Yet if I try to run an app build using the BlitMask technique it will crash if I use more than about 14 images.
One way to avoid might have been to only display a subset of images. But then I'd have to do a blitMask.update(null, true) after setting images to visible=true and visible=false. That takes way too much processing time, resulting in major stuttering.
The difference seems to be in the Bitmapscroller not having to add the images to the stage, it loads them into memory (using canvas.copyPixels(photos, new Rectangle(0,0,1,1), zp)) and then blits them to a 768x1024 canvas. The BlitMask solution seems to necessitate adding a DisplayObject containing all images to the stage, thereby crashing iOS apps if the DisplayObject containing the images becomes too large. I tried adding the BlitMask itself to the stage & then do a scrollX, but that will only work after adding the aforementioned DisplayObject containing all images.
Still, maybe I am missing something. Is there any way to use a BlitMask with a large amount of images on iOS?