You could try setting a new css class in the cell element of the disabled row and mark the style with !important so it will not be overridden by the selected row style.

Here is an example of such workaround. Assign the new style to the disabled row for example, the row with key value 3 and WebGrid column ID, Name, Author, Category during the WebGrid AfterInitialize client side event.

function wgTest_OnAfterInitialize(controlId)
{
	var wgTest = ISGetObject(controlId);

	var rowStruct = ["ID", "Name", "Author", "Category"];
	var rowObj = wgTest.RootTable.GetRowByKeyValue("3");
	for (var i = 0; i < rowStruct.length; i++)
	{
	    rowObj.GetCell(rowStruct[i]).CellElement.className += " Disabled-Row";
	}
	
	return true;
}

Mark the css property as important so it will not be overridden by the selected row style:

<style type="text/css">
    .Disabled-Row
    {
        background-color: Red !important; 
        font-style: italic;
    }
</style>