Hello,
attached is a patch which tries to implement the desired feature. It should apply cleanly atop master (b91eafe31a524b391d5cec079cf8f36c2f9d5f30)
With this patch, emacs accepts a new command line argument:
--user-emacs-directory=DIR
which has two effects:
1. it sets the `user-emacs-directory' variable to DIR (instead of the default "~/.emacs.d")