Yesterday I updated the eclass for XEmacs to support site file based initialization similar to what Emacs on Gentoo offers. When I say similar, I mean identical. A rip-off. Stolen. But without the legacy support. Many thanks to the Emacs team for painfully constructing and debugging a good approach to this problem.
With this support in place XEmacs code that isn't provided as an XEmacs package can now register itself so that you don't need any additional configuration to make use of the package. A simple example is for modes to register themselves automatically for the associated file extensions. The Emacs Gentoo project page has a more verbose explanation. It also includes the mechanism to start using this. Add it to your .xemacs/init.el (after you've installed a package that uses this).
So, which packages already use this? Unfortunately the answer currently is none, but patches are pending for app-admin/puppet. Please feel free to file bugs if you would like to see support for this added to other packages.