R5/R6 users can specify the "setup" script that xdm runs by changing the
entry in the xdm-config file (usually in /usr/lib/X11/xdm) to name a
different script; the sample script distributed with the X distribution
simply runs xconsole.  See the SETUP PROGRAM section of the xdm man page in
R6 for precise details.

Pre-R5 versions of the xdm client could be spoofed by in changing xdm's xrdb
resource in the xdm-config file to run a program to change the background
before loading the resources; for example, your /usr/lib/X11/xdm/xdm-config
file may add the line
	DisplayManager.0.authorize: false 
to permit unrestricted access to the display before log-in (beware!) and also
	DisplayManager*xrdb:    /usr/lib/X11/xdm/new.xrdb 
where that file does something (for all connections) along the lines of:
	#!/bin/sh 
	#comes in with arguments: -display :0 -load /usr/lib/X11/xdm/Xresources
	/usr/bin/X11/xsetroot -display $2 -bitmap /usr/lib/X11/xdm/new.bitmap 
	/usr/bin/X11/xrdb $* 
Substitute xloadimage or xv for xsetroot, to taste.  Note that this is a
general hack that can be used to invoke a console window or any other client.

[Thanks to Jay Bourland (jayb@cauchy.stanford.edu), 9/91]