Just use the bag under the saddle. It's clever, practical, and for the weight weenie, adds only a gram or two. Don't allow your Brooks to get soaking sloppy saturated wet, then ride it. Let it dry. It'll be ok as long as you don't ride it soaked. If you do, all bets are off. A reasonable person, using reasonable care and forethought, will cover the saddle when off the bike in a steady rain. There.
Not Ghetto Bikes:
http://i8.tinypic.com/637dms6.jpg
http://i17.tinypic.com/4pjqhkl.jpg
http://i14.tinypic.com/4u3ldnn.jpg