UICollectionViewFlowLayout subclass to vertically align cells at the bottom edge.

gistfile1.m

@interface MTLibraryCollectionViewFlowLayout : UICollectionViewFlowLayout
@end

gistfile2.m

@implementation MTLibraryCollectionViewFlowLayout

- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect;
{
  NSArray *attrs = [super layoutAttributesForElementsInRect:rect];
  for (UICollectionViewLayoutAttributes *element in attrs) {
    if (element.representedElementCategory == UICollectionElementCategoryCell) {
      CGRect frame = element.frame;
      // Get the diff between the ideal height and the actual thumbnail height.
      CGFloat deltaY = self.itemSize.height - CGRectGetHeight(frame);
      // As the element is already centered, the delta should take that into account.
      deltaY = ceilf(deltaY / 2.0);
      // Offset and be done with it.
      element.frame = CGRectOffset(frame, 0, deltaY);
    }
  }
  return attrs;
}

@end

Works great in theory but is still a hack and can't be animated between layouts