RPM-5.0 originally came with LZMA support, and RPM-5.2's XZ support has been backported as well:

http://www.freebsd.org/cgi/cvsweb.cgi/ports/archivers/rpm5/files/patch-xz-support

It could be added to the Mac port as well, if there's any real need for it. Better to just use RPM-5.2 ?

http://trac.macports.org/browser/trunk/dports/sysutils/rpm50