I decided to take another look at this, after I found myself reading a pdf in zoomlock mode and having the pan to the top after every page turn. It turns out, it's actually very easy to have it move to coordinates 0,0 after a page turn. This doesn't take margins into account of course, but at least the new position is a lot closer to where I want to be than otherwise (especially when viewing large pages with a high zoom factor). Since it won't be added to prs+ until after the coming version, here's the bit of code. You can add it to the body of ViewerSettings_x50.js, and then make a custom build.
Code:
Fskin.kbookZoomOverlay.doNext = function () {
this.getModel().doNext();
if (this.isZoomLock) kbook.model.doSomething('scrollTo', 0, 0);
}
Hope this helps alleviate some of the annoyance