You've got to teach TB that that header exists:

   Options  ..  Preferences .. Messages .. Message headers .. 'Add'

   Now you'll get a pop-up to define the header

     Display as: X-HiNet-Brightmail (or whatever, it isn't very important)
     RFC header: X-HiNet-Brightmail
     Uncheck: 'This field is an address list' (because it isn't)
     Check: 'Allow this field to be edited in the message editor'
     Uncheck:  'Display  this  field  in the scrollable part of the
     header pane'

Now you create a filter that checks whether this new header field contains the word spam
Good luck