The only option you have is to write your own SQL into the posts_clauses filter, where you get an array of the JOIN, WHERE, ORDER, etc. clauses that you can alter, add to, remove, etc.
One MAJOR MAJOR note on this, is ALWAYS use the global $wpdb‘s prepare function, which will sanitize all your lovely datas. You don’t want to be allowing any type of injection through your search term custom queries. 🙂