Well, the way I do it is kinda what you want, but I do use policies. Maybe you can use this to figure out how to do it your way.
The NT4 resource kit support tools package <a target="_blank" href=http://download.microsoft.com/download/winntsrv40/rktools/1.0/NT4/EN-US/sp4rk_i386.Exe>(DOWNLOAD HERE)</a> has some tools in it called Logout and Shutdown.
I installed the support tools on the client computers.
I then disallowed access to the Shutdown and Logout buttons/menus on the system. I do this using policies, but you can also do it by using a *.reg file or script at login.
I enforced the use of custom menus/desktop (again, policies or *.reg) and put in a shortcut for logout or shutdown that point to the executables in the support tools directory and use the appropriate switches on the target line to accomplish the reboot, logoff, or power-down. You can get a full list of available switches by querying the executable from the command line. For example, typing shutdown /? on the command line while in the ...\rksupport directory will display the switches you can use with the shutdown executable. With shutdown there is a /R switch for reboot.
Josh
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