UISplitViewController split = ...;
CustomViewController controller = ...;

var root = new RootElement ();
var dvc = new DialogViewController (UITableViewStyle.Plain, root, true);
dvc.ActivateController (controller);

UISplitViewController split = ...;
var about = new StringElement ("About");
about.Tapped += delegate {
    split.ViewControllers = new UIViewController [] {
        split.ViewControllers [0],
        controller
    };
};

var split =(uisplitviewcontroller)rootviewcontroller.parentviewcontroller.splitviewcontroller;
var nav = (uinavigationcontroller)split.viewcontrollers[0];
nav.pushviewcontroller(_yourView_);