Programmatically authenticating with AppEngine from Google+

[glightfoot]

My user is signed in to google+ and using my app -  an app which wants to use webservices provided by an app engine without having to enter their credentials/approvals again to a seperate html page to obtain the app-engine authentication cookie. I'm trying to reduce the number of initial verifcations between my user and the application.

Is it possible from say; the bearer token provided from google.getToken() in google+ to authenticate with App-Engine to produce a working authentication cookie so that my user will be recognised by the user services. I'd like to use the the default user system for user accounts.
It seems this isn't possible with ClientLogin as the tokens do not match, i've not tried AuthSub yet but I suspect it also wouldn't recognise the google+ token.

Any ideas would be great.

g.