[html5] microdata itemref

[Brian Tremblay]

I'm trying to use itemref to include a location using microdata, and I'm 
struggling. It's the wine tasting article at the bottom of the page at 
http://www.tenmercer.com/

Here's a simplified version:

<article itemscope itemtype="http://schema.org/FoodEvent">
   <header>
     <h1 itemprop="name">Franciscan Estate tasting</h1>

     hosted by
     <div itemprop="performer" itemscope 
itemtype="http://schema.org/Person">
       <span itemprop="name">J. Kounellas</span>,
       <span itemprop="worksFor" itemscope 
itemtype="http://schema.org/Winery">
         <span itemprop="name">Franciscan Estate Winery</span>
       </span>
     </div>
   </header>

   <p itemprop="description">Join us for a special wine event featuring
   <a href="http://en.wikipedia.org/wiki/Napa_Valley_AVA">Napa Valley</a>'s
   Franciscan Estate Winery.
   </p>

</article>


What I'm trying to do is associate "Napa Valley" in the <p> element with 
the winery in the header, as that winery's addressRegion. I've tried 
various solutions, resulting in either the document not validating, or 
the microdata not being recognized by Google's rich snippets testing 
tool. I've searched for help, but my Google-fu is weak on this topic. 
Any pointers?

-- 
Brian Tremblay