Hi Kalvin,
You seem to be on the hook for diagrams. I think I remember some discussion with Sparx, here on the forum, that seemed to indicate they were going to beef up diagrams a bit. If memory serves - I could be very wrong about this, so search and check yourself if it might be applicable - the discussion revolved around either allowing stereotypes for diagrams, or allowing tagged values for stereotyped diagrams (and thus for any diagram by extension).
Perhaps there's a viable workaround.
I wonder if you could create something like a text element, give it some custom properties, then make it invisible.
Or perhaps a hyperlink. These are customized text elements, so you might have to figure out how to tweak the background properties; that might involve fiddling with the database record directly, but you're already there. A hyperlink can be made invisible, does not have to show any text, and could (I assume) have the same diagram as its target (i.e. do nothing).
While this is not an attribute of the diagram per se, you could come up with a standard naming convention and then check if an applicable link were present on a diagram. It so, you can then read your properties.
In the meantime, make a feature request directly to Sparx. You'll find the appropriate form by clicking the Feature Request link below the Support link near the bottom of any forum page.
David
