Splicing an extra login table into WordPress

I’m trying to drill down to the exact point where WordPress touches the mysql database to authenticate a given user and password, so I can check another table in addition to the mysql WordPress one. My company needs employees to be able to read our otherwise private only news blog, but we are unfortunately almost completely a Microsoft office, so I need to interface with SQL Server. I can do this easily if I can figure out exactly where it is confirming a user is authentic, but I’ve found isolating this incredibly difficult.

I’ve gotten to wp_authenticate in pluggable.php, but then it goes into apply_filters(‘authenticate’,null,$username,$password). apply_filters is a bit of very difficult to understand spaghetti code in plugin.php. Does anyone know where to go from here?