Thanks for the suggestion, K T.
I'd already read through several of those tips lists, and I was aware that the page turn animation was exclusive to epub documents. I was assuming there would be some way to display a full-screen image in portrait view with an .ibooks file, but perhaps that's not currently the case (which, admittedly, seems like a huge oversight on devices with less-than-10" screens).
I could accept a continuously-scrolling portrait mode if that's a requirement, but I'd at least need to expand the page-images to fill the column, which is where I also wasn't having any luck with. It seems like doing a new section for each page might be a sort of hack, to get around this as well, although it might preclude re-using the images in portrait view.
(I probably also should have mentioned that I realize I could build this as an .epub or PDF book and get it to do more or less what I want, but there would be annoying faux-gutter and borders that I'd prefer to avoid, and iBooks Author otherwise seems to be capable of producing exactly the material I'm looking for with a lot less work than hand-coding an ePub file.)
Okay, that was what seemed to be the case, and the answer I was looking for someone to give but wasn't finding; "No, you cannot turn off continuous scrolling in portrait view."
(As for hacks, I'll take anything that works--I was willing to manually hack compiled Kindle files to get around a stupid bug in Amazon's Kindle exporter.)
I'll try rephrasing my question, with the understanding that portrait view will always be continuously scrolling:
Is there any way to get an image from landscape view to display at roughly full-screen size (width) in portrait view? Or will they always be thumbnails with the assumption that portrait must accomodate variable-screen-size devices?
(If the answers are "No" and "yes", the solution would be to either just lock it in landscape or abandon IBA in favor of a fixed layout EPUB. Well, that, and file a feature request with Apple, since it seems like this is something you should be able to do.)