After hours of trial and error I've found a fix: I applied an 'innerCover' class to all the pages, and used the following code to remove that class from the pages that are being turned at the start of the turn, so that they look like an inner page as you make the turn. It's a bit hacky but it works:
$('.flipbook').turn({
when: {
start: function(e, page, view) {
var book = $(this),
currentPage = book.turn('page'),
pages = book.turn('pages');
for(var i=3; i<pages; i++){
if(i==page.page || i==page.next) {
$('.p'+i).removeClass('innerCover');
} else {
$('.p'+i).addClass('innerCover');
}
}
}
}
});