destroot.violate_mtree doesn't work?

I'm still trying to port Amanda (www.amanda.org) to Macports. Amanda wants to install its own dumper in /opt/local - not great, but it does this under FreeBSD too. In order to accommodate this I wanted to set: destroot.violate_mtree yes per the Macports guide at http://geeklair.net/new_macports_guide/. However, 'port' doesn't seem to like this directive: Can't map the URL 'file://.' to a port description file ("invalid command name "destroot.violate_mtree""). Any idea what I'm doing wrong? - Julian