In addition to looking at Axure, Omni should check out tools like http://rapidrabb.it/
This is very much like producing a diagram in OG, with options to add attributes to elements placed on the page.
The more I think about how we extend OG to do more and more interface/interaction design document, the more I wonder if OG is the right tool for the job. I don't think this is necessarily within scope for OG, and I've never heard Omni come forth and say this space is important.
Having said that, however, I can see OmniGroup creating a new product based on OG that does more than OG can ever do with regard to extending it's ability to create interactive elements in an exported prototype. If RapidRabbit produced their tool as an affordable standalone Mac app, I would seriously consider using it instead of OG. I would do the same if Axure produced an affordable Mac version.
So, I guess the ball is now in Omni's court. Do they think we're a sizable enough market to target as one of the primary use cases for OG? If they do, can they envision a product roadmap that takes our needs into account, e.g. as a paid product add on, plugin, etc. or by creating a separate product?
If Omni were interested in taking us seriously, I would be happy to participate in earnest to the discussion. When I design interfaces, I spend something like 50% of my time drawing on paper and the other 50% in OmniGraffle.