Export via Scripting Bridge / Modifying the selection of OmniGraffeDocument programatically

Hi all,

Using the Scripting Bridge in 10.5 and OmniGraffle 5 Pro I'd like to do the
following:
I'd like to export the individual parts of a user-selected Illustration
within a Document. The Illustration should not be exported as one entity (I
got this working already), but as individual images. The reason for this is
that I plan to use the individual parts for animation purposes in PowerPoint
2008 / Keynote 08. The Problem now is the following:
The export using the Document works:

self.selectedItems = self.selectedOmniGraffleWindow.selection;
NSString *tempDirectory = [openPanel directory];
NSLog(@"Chosen directory: %@",tempDirectory);
for (int i=0; i< [self.selectedItems count]; i++)
{
    OmniGraffleGraphic *item = [self.selectedItems objectAtIndex:i];
    NSString *fileName = [NSString stringWithFormat:@"/%d",i];
    NSString *finalDirectory = [tempDirectory
                   
stringByAppendingString:fileName];
    NSURL *exportDirectory = [NSURL URLWithString:finalDirectory];
    [currentDocument saveAs:@"png" in:exportDirectory];
          // This does work - It just exports the whole selection!
}


However, If I replace the call to currentDocument with item, as it was
originally intended to iterate over the different graphics in the selection,

[item saveAs:@"png" in:exportDirectory];

it fails with an exception.

My questions:

1. Is it mandatory to use the OmniGraffleDocument class to export items?

The AppleScripts regarding export I've seen so far seem to indicate that.
However, in the generated header file used by the Scripting Bridge
(generated using the sdef command in Terminal), both OmniGraffleDocument and
OmniGraffleGraphic inherit  from OmniGraffleItem, which supplies the saveAs:
in: method. 

2. If it is mandatory to use OmniGraffleDocument, is it somehow possible to
modify the selection of the Document?
With that, I also would get the desired effect of iterating over the Array
of selections and exporting the items one by one.

I appreciate any comments!

Thx in advance!

Toby