You need a language in which writing down any point requires you to also write down every point which could correspond to that under taxicab rotation/reflection. This means every time you write down a point, you also write down eleven other points (in three dimensions). Testing whether any two points are equal is equivalent to a set comparison (the two sets contain all the same points). Testing whether any sets of points are equal extends from this.
There's your maximal language of k-ominoes. Enjoy.
Worth mentioning it doesn't actually matter that the point sets are contiguous. Equivalences of (contiguous) k-ominoes is are special cases of equivalences of point sets.
You need a language in which writing down any point requires you to also write down every point which could correspond to that under taxicab rotation/reflection. This means every time you write down a point, you also write down eleven other points (in three dimensions). Testing whether any two points are equal is equivalent to a set comparison (the two sets contain all the same points). Testing whether any sets of points are equal extends from this.
There's your maximal language of k-ominoes. Enjoy.