The only solution I found is to call the api directly within the list.tpl.
It is working but i'm not sure it is the most efficient way.
Here what is did :
getting user_id of the first message, and the looking for the avatar, if it is >0 then i display it after, all within <?php tags
$userdata = phorum_api_user_get($PHORUM['TMP']['MESSAGES']['user_id'],true);
$profile=$userdata['mod_user_avatar']['avatar'];
Your thoughts ?