[Zope] Seeking tips for persistence in external methods

Hi, I'm having a tree structure which is defined in a flat file via simple x.y.z tokens. I wrote an external method which gets passed such a token, parses this file and returns a dummy method in order to be able to represent this hierachy in zope via the tree tag. Now, this works, but naturally takes very long because it parses the file with every invocation from the tree tag. Now I can pre-build the whole tree out of objects, but the question is how to store that tree. I don't want to use zopes ZODB, because the tree changes potentially every hour. As I found out, pickling from an external method seems to be not an option. anybody any ideas? TIA, oliver