I don't think that is possible, because Scout is a package that supports multiple drivers. Not all drivers support this functionality, so you can't make it available as a generic function for Scout.
In general, if you want to have such functionality you should create multiple indexes. One for all the fields of your thread model and one for only the title. So instead of making the search column specific, you make the index specific. However, I'm not sure if Scout supports creating multiple indexes for one model though.