On a Silverstripe instance running MySQL I need to create a module that talks to a read only remote MSSQL database table. All of the talk here is about connecting Silverstripe to MSSQL and that's not what I need to do.

My best guess is that I have to use whatever the best php connectors are to set up and talk to MSSQL manually and not use some sort of mixed ORM. Would I be right?

Nah. Doable no matter the server (provided you have the correct adaptors installed - but MySQL is default, so should always work)

SQLQuery and DB::query() exist for more low level access, most likely what you're looking for.

If it's on the same server (and the user has permission) just fully qualify your queries.

If it's a separate server, open a new connection. Be careful about overriding the default connection though - be sure to name your connection.

$db = DB::getConn($name) ?: DB::connect($params, $name); //see API for more info
$db->query($sql);