The idea I am getting at here is that this would be *identical* to what we do in the existing coordinate classes.  When you define a new class, you give it the decorator `coordinate_alias('somename')` (or just do ``coordinate_alias('somename', MyCordinateClass)``), and that registers that name in a place that `SkyCoordinate` would look in whenever it needs names of coordinate systems.

You can see examples of how this works by looking in the current `astropy.coordinates.buildtin_systems` module.  All of the builtin classes are written to use the same API that a user would use to define their own custom coordinate frame.

I'll add a comment below this in the document indicating that this can mostly be reused from the existing code base.