Sandbox environment for Emacs configuration

[Christoph Scholtes]

On Windows, I want to create a sandbox environment for Emacs to run in 
with an experimental configuration without affecting my regular setup.

I create site-startup.el in D:/devel/emacs with the following code:

(setq user-emacs-directory "D:/devel/emacs/dotemacsdotd")
(setenv "HOME" user-emacs-directory)

then invoked Emacs with

emacs -L D:/devel/emacs

where D:/devel/emacs just contains the site-startup.el.

However, Emacs still starts up with the init.el from ~/.emacs.d instead 
of D:/devel/emacs/dotemacsdotd/init.el.

My thought was that site-startup.el was loaded first, changes 
user-emacs-directory and HOME and then init.el is invoked from the new 
user-emacs-directory. Ultimately Emacs is running with dotemacsdotd as 
the new home directory.

Why does this not work? Is there a better way to do this?

Christoph