There doesn't seem to be a standard way of installing *.el files for xemacs, such for example an eclass.
Two rudimentary eclasses (xemacs-elisp.eclass and xemacs-elisp-common.eclass are now available from the emacs overlay, along with an ebuild for ebuild-mode 1.5 using them. I expect more work is needed before they will be useful in general, but perhaps brave people already want to play with them. See http://overlays.gentoo.org/proj/emacs/wiki/WikiStart for the overlay details.
Closing this bug as the eclasses have been moved to portage CVS some time ago. I expect that more functions may be needed in them, but I'd rather work on this based on actual demand by specific eclasses.