Comment 19 for bug 777425

Good to hear there is finally some progress on this.
MATE's vmname is Marco
It can be verified by looking at this source file: https://github.com/mate-desktop/mate-window-manager/blob/master/src/core/display.c

    meta_prop_set_utf8_string_hint (the_display,
                                    the_display->leader_window,
                                    the_display->atom__NET_WM_NAME,
                                    "Marco");