register a region with a marionette sub-app after root app instantiation

gistfile1.js

// This code assumes you start all your sub-apps during the root-app's init
// phase.  If not, this same logic can be bound to a different event (ex. start).
rootApp.on("initialize:after", function () {
    vent.trigger("system:default:region:set", rootApp.body);
});

// Then in some other sub-application
define([
    'marionette',
    'mysupervent'
], function(Marionette, vent){

    var someApp = new Marionette.Application();

    var regDefaultRegion = function(reg) {
        someApp.defaultRegion = reg;
    };

    // As long as this someApp has been initialized when the event above is
    // triggered, then rootApp.body will be passed as `reg` to the
    // regDefaultRegion method above.
    someApp.addInitializer(function(options) {
        vent.bindTo(vent, "system:default:region:set", regDefaultRegion, this);
    });

    // As long as the component in question uses the same instance of supervent,
    // you can now get and use this region from any piece of your application by
    // doing `var mainReg = vent.reqres.request("app:request:region:default");`
    vent.reqres.addHandler("app:request:region:default", function(){
        return someApp.defaultRegion;
    });

    someApp.start();
});