If you try to upgrade from 1.1.x to 1.2.0, you cannot use a DB user that only has access to mantis' db. The upgrade process won't even start, complaining that it doesn't have access to the template1 db.

The problem comes from this section in admin/install.php:

<!-- connect to db -->
<tr>
<td bgcolor="#ffffff">
Attempting to connect to database as admin
</td>
<?php
$t_db_open = false;
$g_db = ADONewConnection( $f_db_type );
$t_result = $g_db->Connect( $f_hostname, $f_admin_username, $f_admin_password );

if( $t_result ) {

    # check if db exists for the admin
    $t_result = $g_db->Connect( $f_hostname, $f_admin_username, $f_admin_password, $f_database_name );

The first connect will fail, because the database name is not provided. It's actually an useless check. The only result worth checking is the second one.

i.e. I have a mantis user that is the owner of the mantis database: it can do anything on that db, but it can do nothing on other databases.

There's no reason to force the upgrade process to run as superuser: for one, it might not be possible in some environments where the DBA won't allow it.