This script should do it:

    (function () {
        var overrideCtx = {};
        overrideCtx.Templates = {};	
    	overrideCtx.Templates.OnPreRender = function(ctx) {
    		if (!ctx.inGridMode)
    		{
    			return;
    		}
            var statusField = ctx.ListSchema.Field.filter(function(f) {
                return f.Name === 'TrackingStatus';
            });
            if (statusField.length > 0) {
    			for (var i = 0; i < ctx.ListData.Row.length; i++)
    			{
    				if (ctx.ListData.Row[i].TrackingStatus == "")
    				{				
    					ctx.ListData.Row[i].TrackingStatus = "In Progress";
    				}				
    			}            
            }
        };
    	
        SPClientTemplates.TemplateManager.RegisterTemplateOverrides(overrideCtx);
    })();

The important thing to note is that the values are not saved to the list until you make some change to the row.

**UPDATE:**
There is another approach, that involves attaching the event to the jsGrid control, which checks for the field value and updates it if it's empty. 

    SP.SOD.executeFunc('clienttemplates.js', 'SPClientTemplates.TemplateManager.RegisterTemplateOverrides', function() {
            var alreadyAttachedEvent = false;
            SPClientTemplates.TemplateManager.RegisterTemplateOverrides({
                OnPostRender: function(ctx) {				
                    if (ctx.enteringGridMode || !ctx.inGridMode || alreadyAttachedEvent)
                      return;
    				var jsGrid = null;
    				for(var v in g_SPGridInitInfo)
    				{
    					var jsGridContainer = $get("spgridcontainer_" + g_SPGridInitInfo[v].jsInitObj.qualifier);
    					var jsGrid = jsGridContainer.jsgrid;
    					break;
    				}
    
    				jsGrid.AttachEvent(SP.JsGrid.EventType.OnCellEditCompleted, function(args) {
    					var rec = jsGrid.GetRecord(args.recordKey);
    					if (!rec.properties["TrackingStatus"].localizedValue)
    					{
    						var update = SP.JsGrid.CreateUnvalidatedPropertyUpdate(args.recordKey,'TrackingStatus',"In Progress",false);
    						jsGrid.UpdateProperties([update], SP.JsGrid.UserAction.UserEdit);
    					}
                    });
    				alreadyAttachedEvent = true;
    			}
    		});
     });