I took it up with Gerd, D.Love and RMS. They agreed my proposal to
1) Using <emacs-custom(a)sunsite.auc.dk>. as a shared Emacs/XEmacs forum
for discussion changes to customize.
I have added Gerd and D.Love to the list, I think all the XEmacs
customize maintainers are already on it, if not, subscribe by sending
a mail to <emacs-custom-subscribe(a)sunsite.auc.dk>.
2) Letting me try to bring to two code bases closer to each other. I
got more or less free hands for that.
My goal is that the only differences should be either due to
differences in the underlying mechanism (mostly hidden in cus-face.el
and the widget abstraction) or different UI style preferences (mostly
manifested as different standard values for user options).
...
I'd like to know if XEmacs people agree to these ideas, and if so, how
changes from me to the XEmacs customize implementation should be
handled.