Accessing entity relationships to supertypes

I have a macro for generating meaningful Constraint Names on relationships. 

I'm using the following, which works fine for all ordinary relationships:

    For Each relp In ent.ChildRelationships

However, this does not include relationships to supertypes (ie. the FK relationship from a subtype PK to the supertype).
I cannot see any property on the 'Entity' object for accessing these relationships within the macro.

Does anyone know how to access them?

I've looked at '
SubTypeClusters' on the Model object, but cannot see any attributes on SubTypeCluster for setting the constraint name or getting a Relationship entity..