Setting managed file default value from ajax request
So after digging through Drupal core code I arrived at this solution:

In my ajax callback I load the actual file, set some managed file settings in the managed_file array, and then physically call the file_managed_file_process() function to setup the managed file display field array elements.

function load_image_callback($form,&amp;$form_state) {
    // Load node from the selected value
    $node = node_load($form_state['values']['slide_nid'];
    $file = file_load($node-&gt;field_main_image[$node-&gt;language][0]['fid']);

    // Set the managed_file default value to the nodes main image file
    $form_state['complete form']['slide']['slide_image']['slide_fid']['#value'] = array('fid' =&gt; $file-&gt;fid);
    $form_state['complete form']['slide']['slide_image']['slide_fid']['#file'] = $file;

    // Process the managed file field with the set element
    $form_state['complete form']['slide']['slide_image']['slide_fid'] = file_managed_file_process($form_state['complete form']['slide']['slide_image']['slide_fid'], $form_state, $form);

    // Return the updated slide image wrapper to replace the existing one
    return $form_state['complete form']['slide']['slide_image'];
}