Rendering text from other documents' objects

What's the proper way (if any) to get a piece of text rendered in the context of another document? One of my users has created aggregate pages that include fields of objects from other documents, rather like XWiki's blogging feature. However, when one of these object fields references attachments in their containing documents, these references (via the {image} macro) fail when rendered in the context of the aggregate document. I have tried replacing the direct reference to the field with $includedDoc.getRenderedContent($includedObject.text), and a few other tricks, like XWikiContext.setDoc with the XWikiDocument object of the objects' home document, without success. I can, of course, have him qualify the references to the attachments so that they are absolute (at least, relative to the wiki rather than to the referring document), but that's not nice. brain[sic]