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

[Jeff Kowalczyk]

> Make an External Method. Call it on the folder and 
> give it as parameter the meta types your want to allow.

I've created changeAllowedMetaTypes.py and installed it as an External
method.

from string import join

def changeAllowedMetaTypes(self, mts):
	"""set _allowed_meta_types to mts"""
	self._allowed_meta_types = tuple(mts)
	self._p_changed = 1
	return '_allowed_meta_types changed to ' +
join(self._allowed_meta_types) + ' on ' + self.id

I'm calling it like
http://localhost:8080/myFolder/Images/changeAllowedMetaTypes?mts:list=Im
age&mts:list=Folder

And it's returning '_allowed_meta_types changed to Image Folder on
Images', so the call seems to be succeeding. However, the folder Images
still shows the full metatype list when I return to it. Am I missing
something?