override defcustom

[Bruce Ingalls]

I'd like to override the system defcustom defaults, in a way that users 
can override my override.

Example:
{ignoring that modern X/Emacs now properly default 
font-lock-maximum-decoration = t}

I can set font-lock-maximum-decoration to t by
(custom-set-default font-lock-maximum-decoration t)
 "Toggle max colors for all modes."
  :group 'emacro
  :type 'boolean)

This works, except that now users who don't want max decoration, must 
now set it to nil in 2 places: the emacro and the original font-lock groups.

I tried some alternatives, such as

customize-set-variable()
and
custom-set-default()

but they don't byte-compile or eval properly.
Is it possible to customizably set these variables?
Thanks ahead, Bruce