Append data to all fields of type x I want to add additional content/code to all existing fields of type x. I'm unsure of what options I have for achieving this. I initially thought that hook_form_alter on all *_node_forms would be ideal but this won't cover all instances of the field in the form.

Is there a hook I can use to get all instances of a specific field type?
Depending on what you're trying to add/change hook_field_widget_form_alter() might be an option. It's invoked whenever a field widget is built and you have access to context data so you can easily determine which field type/entity type/bundle you're dealing with.

function hook_field_widget_form_alter(&amp;$element, &amp;$form_state, $context) {
  // Add a css class to widget form elements for all fields of type mytype.
  if ($context['field']['type'] == 'mytype') {
    // Be sure not to overwrite existing attributes.
    $element['#attributes']['class'][] = 'myclass';
  }
}