This seems like exactly what I am looking to accomplish! Would you be willing to share the code for the custom device type and/or the xml save file for EventGhost? I am very interested in mimicking your setup, but I do not have any knowledge on HTTP Get calls or what plugins to enable in Eventghost to get all of this working with SmartThings.
If I could get a general grasp on the process of getting XBMC/EventGhost talking with SmartThings, I would be more than willing to write up a tutorial to assist others in need.