[Zope] Acquire variable for ZCatalog indexing?

I asked this previously, and got no replies. I'll give it one more shot. When making a Z Catalog of a certain class of object, is it possible to acquire a variable from parent objects? I'd like to define category_id as a property of a folder, and then have my Z Catalog pick up that property when it is indexing the objects in the folder. Is this currently possible? If not, wouldn't it be a handy enhancement? :)