I found a way to fix the problem, but I needed to modify ZWiki a little in order to do it because of an issue with the way it uses aq_parent. Here's what I did, in case you are interested:

I modified the renderText() method in the AbstractPageType class in ZWiki/pagetypes/common.py
I changed:
        p = p.__of__(page.aq_parent)
to:
        wiki_base = kw.get('WIKI_BASE', page.aq_parent)
        p = p.__of__(wiki_base)

in CMFContentPanels/skins/contentpanels/viewlets/viewlet_zwikipage_body.pt
instead of:
        python:here.render(here,request)
do:
        python:here.renderText(here.text(), here.pageType(), WIKI_BASE=here.getWikiBase())

I also added a script called getWikiBase to the root of my Plone site:
        wiki_base = context.aq_parent

        if context.meta_type == 'ZWiki Page':
          while context.wiki_url() != wiki_base.absolute_url():
            wiki_base = wiki_base.aq_parent

        return wiki_base

I hope you find this useful..

Cheers,
Edwin