I think browser vendors may be hesitant to implement anything but a clearly defined protocol, because almost by definition a probabilistic model will result in

I think browser vendors may be hesitant to implement anything but a clearly defined protocol, because almost by definition a probabilistic model will result in them occasionally vouching for the wrong sites. Even if they could probably exclude legal liability, it may create PR headaches. As a small fish in the big pond that is the www, I'm also wary of any solution that may end up giving the handful of established behemoths an advantage that smaller competitors cannot at least in theory also get (except by growing to similar size). Right now, there's a process that establishes a "good enough" level: EV certificates are within the reach of even small websites, and with my identity being known and proven, I'd argue that they actually track pretty closely to the actual "truth" of the respective trust-worthiness.