Drupal SOLR search for attachments
You can index taxonomy terms with this module:
https://drupal.org/project/apachesolr_term

There is a hook that allows you to specify a custom field for the solr index:
hook_apachesolr_index_document_build_' . [entity type]

Then you can add this field to the solr query by using:
hook_apachesolr_query_prepare

There is a more detailed article on how to do this here:
http://rapiddg.com/blog/adding-customized-fields-apache-solr-index-drupal