Chris,
Would you consider using a cross-platform toolkit for the Celestia GUI? I'm thinking specifically of wxWindows. It's a free C++ library supported on Windows, Unix/GTK and MacOS. It includes an OpenGL widget and lots of other stuff. I haven't used it myself, but it looks very promising. There's detailed info at their website (http://www.wxwindows.org). Using something like this should make it possible to keep the Celestia ports in sync feature-wise.