[Zope] Set _allowed_meta_types on folder instances without creating a new product?

[Jeff Kowalczyk]

> it would be very useful to apply this 
> to existing folders which are specialized in purpose, but
> require no special class of their own.

[Dieter] To apply it to existing folder, you need to redefine their
"all_meta_types" method - in a similar spirit the "mxmObjectManager"
did...

How do I redefine a method (as opposed to setting a property) on an
existing object?

I don't want to subclass Folder into a full metatype for each
combination of metatypes allowed, but I could see making a series of def
methods: scripts or a product module that I could 'attach' as overrides
to instances of regular, already created Folders as needed. Is that
possible?