An excellent question that, to me, demonstrates the uselessness of the AppActivate function. To dynamically find the window title or PID you surely need a WinAPI function like EnumWindows, but you can't get to WinAPI from VBS. So if you really have to do this from VBS, you need to write a COM object in a language that can use WinAPI - but then, you suddenly have the whole of WinAPI available so why would you want to limit yourself to the VBS AppActivate function? Hence - a useless function in my book.
Sorry for not much help Charlie.