[Zope] How to catalog all objects inherited from a certain class?

Hello, I am looking for a way to catalog all objects inherited from my class. Objects may have different meta_types so I can't use obj_metatypes=['MyClass']. I found that ZCatalog uses ZopeFind for the search. ZopeFind has a parameter obj_expr that gets evaluated on the fly. I would like to say something like ZopeFind(..., obj_expr='isinstance(self, MyClass)' ) but I don't know how. Any insight? Thanks, Igor