I can believe that it's better for people to coordinate movement and breathing without buying into dubious metaphysics.

But it's not only about coordinating movement and breathing. If you reduce yoga to those elements you lose something.

Modern body work usually is also about things like authentic expression of emotions. It's about constantly discovering new ways to move your body.

I was looking for something which was harmless (probably), difficult, and possible to evaluate objectively-- the first makes it rationalist, and the second and third make it a potential sport.

I had to check the dictionary. Webster has two meanings for sport: "a contest or game in which people do certain physical activities according to a specific set of rules and compete against each other" and "a physical activity (such as hunting, fishing, running, swimming, etc.) that is done for enjoyment".

I don't think that competition is needed.