Im finding a KLICKfix Bracket for Bromptons that is topped by a tube.

thereby KF bar bag and Ortlieb Bar bags can be fitted to the Brompton front bag clip,

as well as anything else that is made to fit a round tube.

seen here :http://picasaweb.google.com/ksuzuki....74207078479602

Of note KLICKfix Vario Rack which uses a bracket which you may fit to the BF steerer stem and carry what waterproof Rucksck you may own
or a River-portage pack or Dry-bag..
http://www.klickfix.de/ see new products ...