There should be no problem with this. After all, even the "greeter" is just an X application. Depending on which login manager you use (xdm/kdm/gdm/whatever) the details of getting your Tkinter app to actually be run will vary, though. In gdm, it looks like adding it to the file /etc/X11/gdm/Init/default may be the ticket.
It is probably best to run
app.tk.call("rename", "send", "")
in your program, for the reasons outlined in the send(n) manpage:
SECURITY
The send command is potentially a serious security loophole. On Unix,
any application that can connect to your X server can send scripts to
your applications. These incoming scripts can use Tcl to read and
write your files and invoke subprocesses under your name.
Jeff
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