This plugin contains a custom Similarity class that can be used in an index:

curl -XPOST 'http://host:port/tweeter/' -d '
{
  "settings": {
    "similarity": {
      "index": {
        "type": "org.elasticsearch.index.similarity.CustomSimilarityProvider"
      },
      "search": {
        "type": "org.elasticsearch.index.similarity.CustomSimilarityProvider"
      }
    }
  }
}'

Enjoy