As per a suggestions by jmpp, I've added a new mirror_sites entry for macports.
distname servlet2.3-jsp1.2-tc4.x
master_sites macports:jberry
That would fetch the file that's currently in /distfiles/jberry
To make things easy I also added a :nosubdir url at /distfiles/general/, so you can drop tarballs in the /distfiles/general/ directory and access them like such:
distname some-random-file.tar.gz
master_sites macports
Please don't drop tarballs in the /distfiles/ directory directly, though.
Unfortunately until 1.4 is released we can't really use this in existing Portfiles. But perhaps we can get it out the door soon, and start using this everywhere we need to host our own source drops.