yes, meta-packages are possible. "lxqt" is such a meta-package. do "pacman -Sg lxqt" to see, which packages it contains.
these meta-packages are introduced by arch linux and usually not modified by manjaro. personally, i see a problem when we want to combine "cantata" and "mpd" in a meta-package. somebody has to do it and maintain those (because dependencies might change in the future). @Ste74 already maintains a lot of packages (and does manjaro gnome). i do not want to make him maintain many more meta-packages.
in my opinion, this problem needs to get solved upstream in calamares. i am sure when other distributions start to use the new netinstall feature, they will quickly discover the limitations we are talking about.
i will ask about that in the calamares issue tracker.
i have not looked at the package selection the way you look at it. i will probably re-order the package selection completely.