Example deploy_services diff logic. Most of the hacks needed to make this work have been documented. This is a PoC, not production ready.

deploy_sevices_client.module

<?php

/**
 * Implements hook_menu().
 */
function deploy_services_client_menu() {
  $items = array();
  //$items['job/%wf_job/diff/entities'] = array(
  // dev hack
  $items['job/123/diff/entities'] = array(
    'title' => 'Content diff',
    'description' => 'Provide content of the job',
    'page callback' => 'deploy_services_client_render_entity_diff',
    'page arguments' => array(1),
    // FIXME: Use real access checks.
    'access callback' => TRUE,
    'type' => MENU_LOCAL_TASK,
  );
  return $items;
}

function deploy_services_client_base_remote_url(WfJob $job) {
  return variable_get('deploy_services_client_endpoint_url');
}

function deploy_services_client_get_diff($url, $job) {
  // This should be a services call, but it is a stub for tesing.
  $response = deploy_services_diff_plan("job-123");
  return $response;
}

/**
 * Do not name this function deploy_services_client_entity_diff - it conflicts with hook_entity_diff().
 */
function deploy_services_client_render_entity_diff($job) {
  $remote_url = deploy_services_client_base_remote_url($job);
  $diffs = deploy_services_client_get_diff($remote_url, $job);
  // TODO use a render array and themeing here.
  $header = array('&nbsp;', t('Old Revision'), '&nbsp;', t('Current Revision'));
  drupal_add_css(drupal_get_path('module', 'diff') . '/css/diff.default.css');
  $output = '';
  foreach ($diffs as $diff) {
    $output .= '<h2>' . l($diff['info']['label'], $remote_url . $diff['info']['url']) . '</h2>';
    foreach ($diff['diff'] as $field => $data) {
      $output .= '<h3>' . filter_xss($diff['info']['fields'][$field]['label']) . '</h3>';
      $output .= theme('table', array('header' => $header, 'rows' => $data));
      //$output .= '<pre>' . print_r($data, TRUE) . '</pre>';
    }
  }
  return $output;
}

// ALL THE CODE BELOW HERE GOES INTO THE deploy_services MODULE.

module_load_include('inc', 'diff', 'diff.pages');

// This will be needed in the real implementation
// module_load_include('inc', 'deploy', 'deploy.manager');

function deploy_services_diff_entity($type, $new_revision, $old_revision = NULL) {

  $entity_info = entity_get_info($type);

  $bundle_key = $entity_info['entity keys']['bundle'];
  $revision_key = $entity_info['entity keys']['revision'];
  $id_key = $entity_info['entity keys']['id'];

  $results = entity_load($type, FALSE, array($revision_key => $new_revision));
  $new_entity = reset($results);

  if (NULL == $old_revision) {
    $select = db_select($entity_info['revision table'], 'rt')
      ->condition($revision_key, $new_entity->{$revision_key}, '<')
      ->condition($id_key, $new_entity->{$id_key});
    $select->addExpression("MAX({$revision_key})", 'max_rev');
    $old_revision = $select->execute()
      ->fetchField();
  }

  $results = entity_load($type, FALSE, array($revision_key => $old_revision));
  if ($results) {
    $old_entity = reset($results);
  }
  else {
    /*
    If the old entity can't be found create a new empty one.
    This a hack to make sure the diff shows new entities properly.
    */
    $entity_values = array(
      $id_key => $new_entity->{$id_key},
      $revision_key => NULL,
      $bundle_key => $new_entity->{$bundle_key},
    );
    $old_entity = entity_create($type, $entity_values);
    field_attach_load($type, array($new_entity->{$id_key} => $old_entity), FIELD_LOAD_REVISION);
  }

  $context = array('entity_type' => $type);

  $uri = entity_uri($type, $new_entity);

  $diff = array();
  $info = array(
    'label' => entity_label($type, $new_entity),
    'fields' => array(),
    'url' => url($uri['path'], $uri['options']),
  );
  $entity_diff = diff_compare_entities($old_entity, $new_entity, $context);
  foreach ($entity_diff as $field => $data) {
    $info['fields'][$field] = field_info_instance($type, $field, $new_entity->{$bundle_key});
    list($old_field, $new_field) = diff_extract_state($data);
    $diff[$field] = diff_get_rows($old_field, $new_field, FALSE);
  }
  return array('diff' => $diff, 'info' => $info);
}

/**
 * Provides a dummy list of entities.
 */ 
function deploy_services_get_dummy_entities() {
  $entities = array();
  $entities[] = array('type' => 'node', 'id' => 17, 'revision_id' => 18);
  $entities[] = array('type' => 'node', 'id' => 17, 'revision_id' => 17);
  $entities[] = array('type' => 'node', 'id' => 12, 'revision_id' => 12);
  return $entities;
}

function deploy_services_diff_plan($plan_name) {
  $diffs = array();

  // Load the real plan - commented out for now
  // $entities = deploy_manager_get_entities($plan_name);
  // Dummy function for PoC
  $entities = deploy_services_get_dummy_entities();
  foreach ($entities as $entity) {
    $diffs["{$entity['type']}-{$entity['revision_id']}"] = deploy_services_diff_entity($entity['type'], $entity['revision_id']);
  }
  return $diffs;
}