Well I kind of just did a practice of how this would work.
Assuming you had a table prefix (WILL NOT WORK WITHOUT A TABLE PREFIX

!) you add this within db_mysql.php:
PHP Code:
$query_string = str_replace(TABLE_PREFIX . 'user', 'a' . TABLE_PREFIX . 'user', $query_string);
Below:
PHP Code:
global $query_count, $querytime;
Now don't actually do this - This is just to illustrate how you could use it. You'd replace the a with the name of the database and a period and it'll work. The problem comes with the key fields like userid, usergroupid. Just a though for people thinking about doing this though