I typically use Iceberg for creating metapackages, but I've always wanted to know how to do it via command line utilities.
productbuild --identitier edu.tamu.clla.hp_designjet_drivers --version 1.0 --package /path/to/pkg1 --package /path/to/pkg2 ... output.pkg
errors with:
productbuild: error: Cannot copy package <path> into product. (Bundle-style package "DesignjetRaster.pkg" is not supported.)
Makes sense since productbuild was intended for creating flat packages, and flat packages cannot contain many things that bundle pkgs can. But can another util be used to create a mpkg?