From this [http://comments.gmane.org/gmane.os.apple.macports.devel/15314 the macports-devel mailing list thread]:

1. Log in: http://build.macports.org
1. Click builders: http://build.macports.org/builders
1. Fill out: property1 Name = portlist , property1 Value = /port name or list of ports/
1. Click 'Force Build'