I'm not sure about the others but the full NetBSD base system can be cross compiled from Linux (and maybe some other unixy systems) using the included build script (I'm not sure what the current state of cross compiling pkgsrc is, likely not as good). You could add or remove things from the base system to learn more about how the build system works or create different install sets or something along those lines, although just looking at what the build system does might be as helpful.