PDFs will always be difficult for any device with a limited screen size. Note that the 1-column split feature is new to PRS+ (the coming release is the first that offers it), and it mimics the behaviour you're talking about - although it does require margin removal beforehand.
PRS+ is mostly Javascript; you can find the source
here. In essence it is loaded on top of the existing firmware.
Personally, I would say these are the major obstacles in coding the feature you describe:
- It requires keeping track of where a user is on a page in zoom lock, which is quite hard. And although it might seem obvious to have a 'top part' and a 'bottom part', if there are margins at the top and bottom, that's not so obvious anymore (coordinate-wise). Also, the sony code that is responsible for these things is quite hard to understand.
- If the user is continously reading in zoomlock mode, presumably the zoom lock buttons are covering part of the page, so we'd have to remove those (not easy). Then of course, the question is how to get out of zoomlock.
- Lastly, it would involve overruling page turn behaviour but only under certain conditions. This would actually be the easiest part.
So in all, I doubt it's worth all the effort...