I noticed that you also opened a support ticket regarding the same question. I already answered it and I will also paste the my answer here in case anyone else has a similar question:
Unfortunately currently there is not such s property that you can use with the RibbonBarMenuItem. I have already logged it, however, in our internal system so that our developers can consider it for future implementation.
All the best,
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed