It works:

st> String subclass: MyString []
st> MyString
MyString

but you have to set the shape:

st> MyString shape
nil

I might add a <shape: #inherit> pragma. It's good that the default is nil, because each class should set its shape manually (or tell that it's fine to use the superclass shape).

> I think elmex's issue is really about the inability to specify variable subclasses
> of any shape; for example

st> MyString withAll: #($a $b)
Object: MyString error: should not be implemented in this class, use #new instead

> Will the aforementioned pragma have a way to make classes traditional ST-80
> variable classes as well?

That's already supported with for example <shape: #pointer>. Shapes were introduced in gst 2.2 and the converter automatically converts variableSubclass: and friends to the appropriate shape pragma. See also the tutorial.