Probably the most surefire way to accomplish this (though still terribly hackish and unsupported) would be to patch -[NSScreen visibleFrame] so that it returns something different... say, by using Unsanity's Application Enhancer tools. Of course, that would only have an effect on Cocoa applications... do do it for Carbon apps too, you'd need to find out what the equivalent function is for them, and patch it as well.