Preprocess fields in 7.x on terms

Thanks a lot, but the code below doesn't work. What did I forget?

function mytheme_preprocess_taxonomy_term(&$vars) {
  $vars['myfield']="Hello, world!";
}

function mytheme_ds_fields_info($entity_type) {
  $fields = array();
  $fields['myfield'] = array(
    'title' => t('My own preprocess field'),
    'field_type' => DS_FIELD_TYPE_PREPROCESS,
  );
  return array('taxonomy_term' => $fields);
}

Or maybe I initially asked a wrong question? The goal is to add a variable in theme preprocess function and have an ability to manage its display position with DS UI.

In 6.x branch I used to write something like that in template.php:

function mytheme_preprocess_node(&$vars, $hook) {
  $vars['myfield']="Hello, world!";

  if (function_exists('_nd_preprocess_node') && variable_get('nd_preprocess_override', FALSE)) {
    _nd_preprocess_node($vars, $hook);
  }
}

...and then adding a new preprocess field with field key 'myfield' thru UI. The most confusing thing is that "preprocess fields" aren't mentioned anywhere in 7.x UI

One more thing I've noticed: mytheme_ds_fields_info is NEVER called, so it's not a term-specific issue. Does it mean I need to create a module and implement hook_ds_fields_info there instead of a theme?

And one more question: the option to create preprocess fields thru UI is removed by some reason or just not implemented yet?

upd: I moved hook_ds_fields_info from theme to separate module and new fields appeared on 'manage display' pages, now I can rearrange them... but field values don't display for terms (node fields display as expected), moving preprocess function also doesn't help.

Finally it seems we have to separate issues:
1) Inability to create preprocess field without creating a module
2) 'there is something strange happening with preprocessing of terms'