In order to support xz compressed manpages all that is needed is an extra line in /etc/man.conf: .xz /usr/bin/unxz -c -d I've tested it with PORTAGE_COMPRESS="xz" in /etc/make.conf and it works.
you need to do a lot more to make it work properly. added it to man-1.6f-r4. http://sources.gentoo.org/sys-apps/man/files/man-1.6f-xz.patch?rev=1.1 http://sources.gentoo.org/sys-apps/man/files/man-1.6f-man2html-compression-2.patch?rev=1.1 http://sources.gentoo.org/sys-apps/man/files/man-1.6f-so-search-2.patch?rev=1.1
*** Bug 307787 has been marked as a duplicate of this bug. ***