Hey keesiemeijer, thanks for the code. I implemented it, but it doesn't seem to work properly. Some sorting does take place, but that is very occasional and I couldn't find a pattern why some posts got moved. Moving of posts from the default arrangement was very rare.
index.php (switched the code I had for pagination for this):
<?php $dates_per_page = 4; get_index_pagination_nav( 'nav-below', $dates_per_page); ?>
I also left the bugged implementation up at the example site.
It would definitely be cooler to use a more optimized code, so I'm all in as far as co-operation :)