Hey, thank you for submitting this patch. Overall I'd say it looks like a really good starting point. I'm sure other developers will add a few more detailed comments soon. I'd just like to mention a few things I noticed on a first glance:
The schema file appears to be identical to the mssql_schema.sql file already present, there is no need to specify a seperate one in that case. Just reuse the existing one. You can do so by setting 'SCHEMA' to 'mssql' rather than 'mssqlnative' in includes/functions_install.php
phpBB does not use a numrows function of any sort because some DBMSs do not support it. So it seems rather odd that you buffer all results in a PHP array just to get the amount of rows even though the API does not have any way of using that information?
I have a few problems with the style/formatting of the code. The indentation seems to be a mix of spaces and tabs. The use of spaces within the code seems rather arbitrary. Some names correctly only consist of lower case characters and underscores, others mix the names with camel case or only use camel case. In order to commit this into the phpBB repository it will have to meet the guidelines explained in http://code.phpbb.com/svn/phpbb/branches/phpBB-3_0_0/phpBB/docs/coding-guidelines.html#code