You mention both CWP and WebDirect here, so which one is it?
Why not just switch to FileMaker 16, which supports Oauth via google natively.
I was looking for solutions both ways actually but custom web publishing is going to be the dominant case.
As for switching to FM 16, the answer is the release date. I haven't even begun to evaluate it to see whether there's a gotcha that would make upgrading unwise until it gets fixed.
FYI, I'm using OAuth with Google authentication in FMS 16 on 2 clients so far and it has been a real successful feature.
What I need is a look at one solution that actually works so I can wrap my head around this stuff.
Maybe it is time to start looking at FM 16