By using the combination of the pin ID and the unique map ID you can get access to the underlying html div element and call the mouseover.
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title></title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
<script src="http://dev.virtualearth.net/mapcontrol/v4/mapcontrol.js" type="text/javascript"></script>
<script type="text/javascript">
var map = null;
var pinID = 1;
function GetMap()
{
map = new VEMap('myMap');
map.LoadMap();
}
function AddPin()
{
var pin = new VEPushpin(
pinID,
map.GetCenter(),
null,
'My pushpin',
'This is pushpin number '+pinID
);
map.AddPushpin(pin);
document.getElementById(pinID + "_" + map.GUID).onmouseover();
pinID++;
}
</script>
</head>
<body onload="GetMap();">
<div id='myMap' style="position:relative; width:400px; height:400px;"></div>
<a href="#" onclick="AddPin();">Add a pushpin to the center of the map</a>
</body>
</html>