The following AppleScript (available for download) is an aggregation of the first two articles on this subject, condensed into a double-clickable application that might even work on PowerBook G4 computers in need of it.
The PowerBook support is a contribution from an article at InsanelyMac authored by their member rdemby (as far as I can tell — at least it's the oldest reference I found using
-- The required FCode gets executed as part of the "boot-command". Its first action is to reset this variable to its default value so the change of the perceived CPU frequency will never survive the next restart – no permanent settings are made.
(A similar technique is used by the "Target Disk Mode" option in the "Startup Disk" pane of the "System Preferences".)
-- "max-clock-frequency" is preferred over "clock-frequency" as it seems to be used by older PowerBooks to describe the nominal CPU speed. No tests have been run yet on such hardware in the given form.
do shell script "nvram boot-command='set-default boot-command dev /cpus/@0
d# 867000000 encode-int \" max-clock-frequency\" 2dup active-package find-property
0= if 2drop \" clock-frequency\" then property device-end multi-boot'" with administrator privileges
tell application "System Events" to restart
PS: On a dual CPU system the clock-frequency will be duplicated by the OS from the first to the second CPU so this script is sufficient to satisfy the installer regardless of the number of G4s.