According to the Introduction:
that will run on any operating system coming with a Java Virtual Machine (JVM) that is compliant with the Sun JVM 1.4 or higher.
This definition includes Win9x. Unfortunately, the native libraries used by IzPack target the WinNT series, Win9x does not support Unicode.
The patch includes the ANSI build output for the natives into the compiler and makes the Java code load the proper library depending on the OS.
If you intend to drop support for Win9x, please clarify your minimum requirements about this.