Adding methods is probably better done in a new base Model class, I would think.
As for anything more complex, I think you'll basically have to setup an entire set of Handlers, like you were setting up a new database the system knew about. That would include Builder, Connection, Forge, PreparedQuery, and Result classes. Each of those would just extend the MySQLi or Postgre files, per your need. This is needed, unfortunately, due to the way that class files are loaded within the database system. Kind of a pain, and probably something that needs to be looked into at some point.