If we derive the speed of light from the Maxwell equations we will find it's a function of the permittivity and permeability of the medium.
Now let's play with the thought that we are living in a substance that behaves like vacuum (eg. no mechanical drag) but have different permittivity and permeability, thus different speed of light.
If we would perform MM experiment inside that substance would it detect wind?
Or in other words can we distinguish between "real" and "fake" vacuum by testing the postulates of special relativity in it?