Richard Stallman recently revived a nearly year-old thread in the emacs-devel mailing list, but the underlying issue has been around a lot
longer than that. It took many years before the GNU Compiler Collection
(GCC) changed its runtime library exemption
in a way that allowed for GCC plugins, largely because of fears that
companies might distribute
proprietary, closed-source plugins.  But efforts to use the plugin API to
add features to another GNU project mainstay, Emacs, seem to be running
aground on that same fear—though there has never been any real evidence
that there is much interest in circumventing the runtime library exception
to provide proprietary backends to GCC.