I'm not sure it is Tk's task... it looks more a problem to be taken to the window manager. Assuming, of course, you are using Tk on X11.
If this is the case, you'll have to resort to the horror that is the ICCCM (Inter-Client Communications Converntions Manual, or How to do IPC over X11 and have nightmares). Or the (cleaner, but narrower in scope) GNOME/KDE WM conventions.
Sorry, out of links at the moment. I'm at my parent's home, on a dial-up (56K) instead of my usual ADSL (320K)
dakkar - Mobilis in mobile