Add the following to your
keycode 66 = Escape NoSymbol Escape
See Remapping Caps Lock with xmodmap doesn't work for explanations.
xmodmap - <~/.Xmodmap to apply your changes immediately. They will be applied automatically when you log in in the future.
(By the way, this is essentially what's going on under the hood in RHEL 6, which is apparently where you got the desire for this feature in RHEL 5. This is what I meant by my deleted comment that this stuff had been nailed down years before. All they've done is put a little GUI glaze on top of an ancient mechanism.)