I think the way modules display this information may actually provide a solid precedent that addresses the dynamic state problem I'm concerned about: filter the subclass list to only include subclasses that come from the same module as the object being displayed, and use the existing showtree/getclasstree interfaces to display the nested hierarchy nicely (the same way modules do).

Then the "... and NNN other subclasses in other modules" would only be appended when there was cross-module inheritance involved.