Diazo XSLT and external document only work with a hardcoded path to the document

I am unable to retrieve an external document that exists in my site using a Diazo XSLT rule:

Basically if I use a relative path to call an external document with Diazo I get:

AttributeError: 'PersistentResourceDirectory' object has no attribute 'getPhysicalPath'

I've documented the details over at stackoverflow: