I just hope Web Intents makes it into the actual release - and in a way that is usable by normal users. Seems to me this should be a priority, instead of putting all that effort into yet another single sign-on fantasy.

If this happens as part of a large integrated experience as you promised that's fine, but I fear that you are taking on too much, going for a complex and over-engineered package that may never be ready or actually usable, instead of concentrating on this one single important thing which really needs to get done.