Changing site-lisp without installing it

[Spencer Baugh <sbaugh@janestreet.com>]

I want to start using site-lisp at my site.  But I'm not sure how to
hack on changed versions of the site-lisp, without installing it.

What would be ideal for me is this:

- There is a normal, installed Emacs, with a site-lisp directory.  My
  users run this and get the installed site-lisp directory.  All normal.

- I run this same Emacs binary in some way, I don't know how, to make it
  use a replacement site-lisp directory, instead of the installed one,
  so that I can hack on the site-lisp, without having to build a new
  Emacs or change the actual installed site-lisp directory.

Now that I say all this, I guess the obvious thing to do is simply pass

--no-site-lisp --directory my/local/changed/site-lisp

and then hack away.

But will that behave correctly?  Are there any footguns there?

(Does anyone actually use site-lisp?  It seems a little under-explored,
but I think it will be really useful at my site, so that people can
easily run "emacs -q" to get an emacs with only the site configuration
which should help with debugging their problems)