The hook_cron_pre_execute($function, &$hook) is invoked just before running the cron function.

Example

Call another function instead of the defined

function hook_cron_pre_execute($function, &$hook) {
  if ($function == 'node_cron') {
    $hook['callback'] = 'mymodule_steal_cron_function_from_node_module';
  }
}

function mymodule_steal_cron_function_from_node_module() {
  error_log('PWNED');
}