Extracting QuickLook Preview from a .graffle file

I am writing a tool and would like to extract the Quicklook Preview (not thumbnail) from .graffle files.  Since there is no public interface to extract QL Previews in general (only to run the QLPreviewPanel), I was thinking of hacking something up to extract the required image from the .graffle file directly.  I see that .graffle files are gzipped plist files and that there is a key QuickLookPreview which seems to contain base64 data or something like that (at least for Omnigraffle 5 files).  

Can someone give me a hint of how to extract this data?  It is an NSCoded version of an NSImage or something like that?


thanks,
Niko