I kind of doubt it. You can just design panels in the normal gui designer and then put them into internal frames. As far as I know the basic gui designer would be sufficient for that.
By coincidence, a co-worker and I were just discussing MDI this afternoon, because I was using SoapUI (which is MDI). We both agreed that MDI is a pretty rough for the users. Internal frames are constrained by the main frame, so the space quickly becomes cluttered and things become hard to find. Users are constantly sorting through them to find what they want. Have you noticed that most apps these days either use multiple top level windows or tabbed panes?