its not exactly the same as forcing the infoWindow to utilize a different anchor, but I was able to pan the map to ensure that the entire infoWindow is displayed using the following code...//set a global fixed anchor for the infoWindow when the application loads (initEditing might be a good place)
map.infoWindow.setFixedAnchor(esri.dijit.InfoWindow.ANCHOR_UPPERRIGHT);
//afterwards wire up the event handling to make sure the map will pan if the entire infoWindow isn't going to fit on the screen
dojo.connect(map.infoWindow, "onShow", function() {
//Determine the upper right, and center, coordinates of the map
var maxPoint = new esri.geometry.Point(map.extent.xmax, map.extent.ymax)
var centerPoint = new esri.geometry.Point(map.extent.getCenter());
//Convert these to screen coordinates
var maxPointScreen = map.toScreen(maxPoint);
var centerPointScreen = map.toScreen(centerPoint);
//Subtract the size of the infoWindow, including a buffer.
//This will show whether the infoWindow would spill out of the current view.
var xDiff = Math.abs(maxPointScreen.x - map.infoWindow.coords.x) - 435;
var yDiff = Math.abs(maxPointScreen.y - map.infoWindow.coords.y) - 285;
//If required, recalculate a new centerpoint which accounts for the infoWindow
if (xDiff < 0) {centerPointScreen.x -= xDiff;}
if (yDiff < 0) {centerPointScreen.y += yDiff;}
//Pan the map to the new centerpoint (in map coordinates)
centerPoint = map.toMap(centerPointScreen);
map.centerAt(centerPoint);
});