i added these overrides. The store.prefetch() does not fire a 'beforeload' event, but it fires a 'beforeprefetch'. In Ext.LoadMask, onBeforeLoad() shows a mask(potentially), and onLoad() Hides it. I also added the refresh as a StoreListener, which is new in 4.1. I believe refresh is similar to the load event, but it is fired in different situations once the data has arrived.

Code:
			Ext.override(Ext.LoadMask,{
				getStoreListeners: function(){
				        return {
				            beforeload: this.onBeforeLoad,
				            beforeprefetch:this.onBeforeLoad,
				            refresh:this.onLoad,
				            load: this.onLoad,
				            exception: this.onLoad     
				        };
				    }
				});