Maybe Apache Solr might be easier to install and maintain than Lucene. Solr uses Lucene and extends it. http://lucene.apache.org/solr/features.html
From the Solr features page: "Solr is a standalone enterprise search server with a REST-like API. You put documents in it (called "indexing") via JSON, XML, CSV or binary over HTTP. You query it via HTTP GET and receive JSON, XML, CSV or binary results."
I used Solr 1.4 in a project a few years ago. It was relatively easy to set up, indexing was fast, and searching was fast and effective.
Solr is a stand alone system, so you could install it independently of Mattermost, just to see how it works. In that case, you'd extract the content from the Mattermost database and send it to Solr for indexing. Solr has a built-in web server, so you could quickly set up a web page for testing the search results.