diamondsandperls:
Remember that  Tk (and, as far as I understand, any GUI) is an event-driven system. Once program execution enters the  MainLoop() function, the only things that count are events:  Button presses, keypresses on the keyboard, mouse movements and button clicks, etc., etc. The keyboard keypresses that would normally be available via  STDIN are  MainLoop events. Keyboard keypresses can go to a  Text widget, an  Entry widget, etc., if the widget is in focus. Otherwise...
(Update: However, I think (without testingsee subsequent post) it should be possible to capture  STDIN data before the  MainLoop is entered provided the  Tk script is launched from the command line. If it is launched via an icon click, we're back in territory unfamiliar to me.)

I know how to re-direct  STDOUT and  STDERR so as to make functions like  print() and  printf() output to, e.g., a  Text box. I do not know how to do anything similar with  STDIN and I am not even sure it is meaningful to talk of doing such a thing (Update: from within the  MainLoop)!

Can you describe in greater detail what it is you hope to accomplish with your program? What is the nature of the data that  STDIN is supposed to provide?