I had this exact same problem and was eventually able to get the functionality you asked for with the following code:

function hideGridMessages(dataRegion, request) {
    var task = new Ext.util.DelayedTask(function(x) {x.hideMessage();}, this, [dataRegion]);
    task.delay(15); // display of messages is delayed by 10 in DataRegion.js
}

var qwp1 = new LABKEY.QueryWebPart({
     schemaName: schemaName,
     queryName: queryName,
     renderTo: 'MyDiv',
     success: hideGridMessages
});

One downside of this method, is that the View Name ribbon will briefly display before being hidden. But in many cases, you won't even notice unless you are trying to see it happen.

-Will