I love using LWIN as another modifier with other keysyms, but it seems that it
only does that with altwin:left_meta_win, but then win becomes meta too..

The following config seems to work, the only problem is that it xev says it
still generates a Super_L and gnome-keybindings-properties sees a <Hyper>, 

partial modifier_keys
xkb_symbols "left_win_mod4" {
    key <LWIN> { [ NoSymbol ] };
    modifier_map Mod4   { <LWIN> };
};