FMS14 does accept URL parameters for WebDirect now, not sure if you knew that or not.
Mike Duncan, Steve Senft-Herrera and I developed a technique to use PHP sessions with WebDirect:
We had done so due to URL parameters being disabled in 13, but this is still a powerful utility for 14 as well.
You might be able to adapt our demos to meet your needs.
Also you might want to consider a simple "auto-login" based on Get(PersistentID), as you could automatically log in users that match a list of people that already logged in. I blogged on that as well: