[Joel]

I'm fairly certain this is indeed possible, however you'll have to bear with me as it's probably going to require some hefty AppleScript to pull it off, and I am sadly not the sharpest tool in the shed when it comes to scripting.

Now the first obstacle is that the .graffle file format isn't the same as Outliner's, .graffle is really just a modified property list (we want to eventually move over to using strict XML for it in the future).

However, notes in OmniOutliner will translate properly into notes in OmniGraffle, so I'm suspecting that if you were to flag the entry as a particular file type, then you can run an AppleScript to change the graphic to whatever is desired after importing it into OmniGraffle.

That said, I should also point out that there's already user-defined metadata for graphics in OmniGraffle, it's just not exposed to the UI but one can use AppleScript once again to set that info, if it's any help.