Hello David,
I had the same problem. I think the child class inherits the graphical
definition from its parent.
So I don't know how to have two different graphical representation for
both classes A and B in your example.
What I did to resolve the problem was to add a third class C that inherits
from A, and define the class A as abstract.
Then you can have your two different representation for your nodes (is not
the best solution but in my case it was fine).
I don't know if it changes too much your model
Cecilia.