[Zope] Automatically adding properties (keywords) to folders

I'd like to set up a folder in Zope with the property that when an object is put into it, the object has a keyword added to it automatically (and when the object is moved out of the folder, the keyword is removed). If you could do this, you would have a nice way to maintain a collection of such folders and have their contents appear elsewhere in a collection of Plone Smart Folders (using keyword criteria). Can anyone tell me how I should best go about doing this? I can imagine writing some python to run once in a while, find all the contents of the folder, add the keywords, etc. But that's far from being an ideal solution. Any pointers would be much appreciated. Regards, Terry