I finally did it. Based upon the plugins I've ported over from Eclipse to compile under GWT I setup a project that provides Observable implementations for GWT and MyGWT widgets but it is open for any other GWT-compatible widget-lib.
A missing thing currently is a 2nd light weight Model-Implementation that works on GWT, I thought about simply providing an Observable implemented ontop of a HashMap. EMF is available and working but I'd like to have a second very light weight implementation which can be used by people not familiar with EMF and only need some GWT related binding.
You can find the project here.