The MD AG has ruled that their interpretation is that that provision does not preclude the state from collecting it's own data or requiring registration.
I would think that registration would be found Constitutional, since in and of itself registration does not preclude the right to possess. You need permits to exercise your 1A rights, to a degree. The problem with a yearly tax on "assault weapons", or high prices on a permit is that they hinder the ability of some people to own the weapons. I would hope that the courts see it that way. Of course, first they have to agree that "assault weapons" are protected under the Constitution because of common use.