I did that and it works well but IMHO the build process is a mess. Why use batch files and shell scripts when ant offers everything you need? Why compile into the directories where you store your source code? Why use the jar command (multiple times) when ant has a jar task?

If you'd organize things a little differently, a "ant release" could automatically compile core and gui, jar everything and copy all relevant files to a release directory. It would be much faster too since the jvm would load only once.