You can access the data inside of loadComplete or beforeProcessing callback. The fist parameter represent the data returned from the server in exactly the same form as it was on the server. Inside of beforeProcessing callback you can even modify the data in any way.
One more good place for extra information is userdata part of the server response (see the documentation). Additional userdata property must be object. The main advantage of the usage of userdata is saving of the value of userdata inside of userData internal parameter of jqGrid. So you can use getGridParam to access the data.