Reorder cells of UICollectionView

Consider a UICollectionView with flow layout and horizontal direction. By default, cells are ordered from top to bottom, left to right. Like this:

1 4 7 10 13 16
2 5 8 11 14 17
3 6 9 12 15 18

In my case, the collection view is paged and it has been designed so that a specific number of cells fits in each page. Thus, a more natural ordering would be:

1 2 3   10 11 12
4 5 6 - 13 14 15
7 8 9   16 17 18

What would be the simplest to achieve this, short of implementing my own custom layout? In particular, I don't want to loose any of the functionalities that come for free with UICollectionViewFlowLayout (such as insert/remove animations).

Or in general, how do you implement a reordering function f(n) on a flow layout? The same could be applicable to a right-to-left ordering, for example.

My approach so far

My first approach was to subclass UICollectionViewFlowLayout and override layoutAttributesForItemAtIndexPath::

- (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath
{
    NSIndexPath *reorderedIndexPath = [self reorderedIndexPathOfIndexPath:indexPath];
    UICollectionViewLayoutAttributes *layout = [super layoutAttributesForItemAtIndexPath:reorderedIndexPath];
    layout.indexPath = indexPath;
    return layout;
}

Where reorderedIndexPathOfIndexPath: is f(n). By calling super, I don't have to calculate the layout of each element manually.

Additionally, I had to override layoutAttributesForElementsInRect:, which is the method the layout uses to choose which elements to display.

- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect
{
    NSMutableArray *result = [NSMutableArray array];
    NSInteger sectionCount = 1;
    if ([self.collectionView.dataSour