We currently have no plans to officially support other database systems.
In fact, I recently upgraded Q2A to use the mysqli_* functions instead of mysql_*. I looked at using PDO, because then in theory it would be simpler to change to another database. However, in practice all database systems use different dialects of SQL and in many cases the queries would need to be rewritten (for example, Oracle doesn't support the LIMIT clause).
If you want to support a completely different database system it's certainly possible! All database access is kept inside the qa-db-*.php files. First you would update the qa-db.php file to connect and run queries using your new database (e.g. replace 'mysqli' with 'pg' for PostgreSQL). It may work find now, but you would probably need to check the other qa-db-*.php files and update the queries accordingly.
Now, MariaDB is a slightly different case. It is supposedly a drop-in replacement for MySQL, so currently there shouldn't be anything we need to do to support it right now. You are right, it may not be like that forever, but I think until a lot of people have moved over from MySQL it will likely stay compatible.
Feel free to give it a try and let me know if you encounter any issues. If there is something we can do easily to support MariaDB without hindering MySQL support, I'll certainly consider it.
Hope that helps explain our position.