Using package.el from site-start

[sbaugh]

Hi,

I maintain an Emacs installation used by many people at my employer.  I
have site-specific customizations in site-start.el, stored in site-lisp.

I would like to be able to use packages installed into site-lisp/elpa
(which is added to package-directory-list) in my site-start.el.  But I'm
not sure how in site-start.el I can activate the packages installed into
site-lisp/elpa without interfering with later customization by users
which might, e.g., change package-user-dir.  If I call
(package-initialize) in site-start.el, that will activate the
site-lisp/elpa packages, but it will prevent users from using
early-init.el to customize the package system.

Also, if possible, I would like to use the package-quickstart-file
functionality to speed up activating the packages in site-lisp/elpa.

Any suggestions on how I should approach this?