theme : div wrapper on each element in a zone to distinguish between first, odd and even elements.

Here is the patch I worked on

I should have though "more reusable" :$
I'm testing your solution, but at first sight it should be fine
I'll tell you right away
cheers
Raphael

Ok, the system is fine but there is a bug.
In composite_views, when you get your references with

$references = _composite_references_preprocess($references, $layout['zones'], FALSE);

They are not sorted by weight. So their positions does not mean anything.
Other point : we choose to put the computing of the position in composite.pages.inc in composite_zones_form_submit in aim to avoid computing positions each time we want to display our composite node. Indeed the computing is quite long, since it needs to study our references array two times : one for sorting, and second to set positions.
So what do you think about it ?
Regards,
Raphael

Here is what I suggest for composite_views :

function composite_view($node, $layout = array(), $references = array()) {
  $composite_content = array();
  $references = _composite_references_preprocess($references, $layout['zones'], FALSE);
  $zone_positions = array();

  $elements = array(); //contains composite elements to dispatch on zones, indexed by zones, then by their weight
  foreach (element_children($references) as $id) {
    $reference = $references[$id];
    if (array_key_exists($reference['zone'], $layout['zones'])) {
      $elements[$reference['zone']][$reference['weight']] = $reference;
    }
  }

  foreach ($elements as $zone => &$zone_elements) {
    if (ksort($zone_elements)) {
      foreach ($zone_elements as $weight => $reference) {
        $output = composite_invoke_referenceapi($reference, 'view', $node);
        if ($output) {
          if (!isset($zone_positions[$reference['zone']])) {
            $zone_positions[$reference['zone']] = 1;
          }
          $reference['position'] = $zone_positions[$reference['zone']]++;
          // Allow themes to modify or override the output
          $output = theme('composite_zone_item', $output, $reference);
          $composite_content[$reference['zone']][$reference['id']] = array(
            '#value' => $output,
            '#weight' => $reference['weight'],
          );
        }
      }
    }
  }
  return $composite_content;
}

With your patch, it works fine. Please tell me if it is fine for you

First, thanks for your concern :) Our team really appreciate it.
Then, to tell you the truth, when we had to determine the position of our references in each zone, our first thought was composite module has a lack of hooks for customizing the theme of a reference. That's why your first proposal seems very good as far as I am concerned, especially if you really do not want to store additional information in the database.
I am pretty sure having the ability to distinguish elements (odd-even) in a zone will be interesting anyway for everyone.
For your second sugestion, as I said, computing the position will be useful I guess, so reducing the time to compute it will help.
Why not implement both of your solutions ?