How to use Scaladoc model or ast to find direct superclasses/supertraits

At the moment, I'm hacking scaladoc in the 2.8.1 trunk to try to add
graphs to scaladoc output. In the inheritance graph, I'd like to show
not only the linearization for a class/trait, but also the declarative
structure; similar to what is done in fig. 12.1 in the Programming in
Scala (2nd ed.) book.

Using the scaladoc model classes (..Entity and related) I don't see an
obvious way of doing this. Do I need to go into the compiler symbol
table and internals, or am I missing something simple? I don't really
want to dive into the compiler structures unless I have to...

Thanks,
Ken