> I've just tested it on Netscape 4.7 and the hack appears to break the page

Yes, any stylesheets including the BMH will be ignored by Netscape 4. If
you need to use \-based hack and still support styles on N4, put the hack
in a separate stylesheet to the 'basic' rules for N4. Or you could try
Caio's hack to hide any bits using the hack from N4 -

  #myelement {  width: 100px; }
  #myelement {  width: 80px;  }
  #myelement { \width: 100px; }
  #myelement { w\idth: 80px;  }
  /* */
  #myelement { padding: 10px; }

the first line is the width for N4. The second line hides the rest from N4.
The third line is the width for Opera 5. The fourth line is the width for
IE/Win Quirks. The fifth line is for standards browsers. The sixth stops
hiding from N4.