Field inheritance (Drupal 7) I would like to make a field that is very similar to the group audience field but only displays certain types of groups.  

As an experiment I hacked up:

og_field_audience_options

and I was able to remove the groups I didn't want but obviously this is not where this code belong.

Is there a way that I can create a new field that inherits most of the behavior of the group audience field and but has a slightly different way of picking which groups to display?  Or do I need to recreate the whole field from scratch?
Strictly speaking, fields can't "inherit", no, but you can almost do it anyway.

Drupal 7 comes with a generic "File" field, and a more specific "Image" field. Technically, one could say that Image inherits from File, but the way it's done is simply that for a lot of it's hook implementations, it just calls back to file.

Example code:

/**
 * Implements hook_field_load().
 */
function image_field_load($entity_type, $entities, $field, $instances, $langcode, &amp;$items, $age) {
  file_field_load($entity_type, $entities, $field, $instances, $langcode, $items, $age);
}


As far as I know, this is as close as you can get.