There is no need for a "rating" since it is not a concept for a multiple player game.

A rating measures the likelihood of you defeating another opponent, based on their rating.

There is no system to predict the likelihood of how individuals would perform in a collective pool all acting in parallel, but there is a way to evaluate your performance independent of who your peers are.

A "rating" is very inaccurate at first, and corrects itself over time.

The "p.a.s.s." method does the same thing. Over time, your participation in a variety of pond games will effect your score. It does not matter if you are playing with an entire pond of perfect 1.0 players, or with a bunch of people who have repeatedly fallen out on the first run around.

Over time, the result will be the same...the scores will all approach their own performance, as surely as "water seeks its own level" in the natural world.