Let's say you had your map object and it's called globalmap (you made as a new google.maps.Map)
This will add a marker on maps' center:
var newmarker=new google.maps.Marker({map:globalmap, position: globalmap.getCenter()});
this will listen to right click on the marker;
google.maps.event.addListener(newmarker, 'rightclick', function(mouseEvent) { alert('Right click triggered'); });
EDIT: Regarding the MarkerWithLabel plugin, I can see that it's extending from google.maps.OverlayView, and passes in a marker in the marker_ property of the object. This means you could still have your way doing (copying from the example):
var marker1 = new MarkerWithLabel({
position: homeLatLng,
draggable: true,
raiseOnDrag: true,
map: map,
labelContent: "$425K",
labelAnchor: new google.maps.Point(22, 0),
labelClass: "labels", // the CSS class for the label
labelStyle: {opacity: 0.75}
});
and then add the right click behaviour as
google.maps.event.addListener(marker1.marker_, 'rightclick', function(mouseEvent) { alert('Right click triggered'); });