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. Select the builders you want to build on (probably all of the buildports-* builders)
1. Fill out: 
  * property1 Name = portlist
  * property1 Value = /port name or list of ports/
1. Click 'Force Build'

You can also click the 'Rebuild' on an individual build page.