My first impression from peeking into the source for Question2Answer is that it seems to be built to co-exist nicely with other OS applications. For example, one can plug in their own code for user database management or for localization - for example if one wanted to use gettext instead of PHP arrays for multilingual messages (even though it's just code filling in, not some OO pattern based style - but that's a different matter all together).
However, there does not seem to be a pre design to allow plugging in search functionality. My question is: if anyone has integrated the sphinx search engine into Question2Answer, then what is the best technique or tips to achieve that.
I checked the docs on creating plugins here: http://www.question2answer.org/plugins.php, and the plugins system seems to accommodate certain types of functionality such as event handlers or content widgets - but not generic functionality or a search module specifically.
Version 1.5 of Question2Answer will support search modules in plugins which allow you to implement a custom indexer and search engine.