Octo, I'm not out there on the 1200k rides yet, but I've done a few double centuries and 300k rides so I know a bit about distance rides.

How much do you think the Al bars are really contributing to a chattery ride feel? I've got Salsa M/A Bell Laps on both my drop bar bikes, and on my fixed gear they're just a plain cloth wrap sealed with shellac. Maybe it's the overall difference in our bikes (mine = steel bike, 28mm mid-pressure tires; yours = Al frame and 23mm hi-pres. tires) that really contributes to the vibration.

If you can fit them, a 28mm tire running under max pressure will smooth out the ride quite a bit. Do you roll tires that narrow when you ride long brevets? I used to run 25mm tires, and switched to 28mm in the same model (Gatorskin) which gave me some extra cushion even at 115psi. Now I'm swapping both 700c bikes to the Vittoria Randonneur Cross 28mm. They only hold 85psi max, but they're such a smooth ride.
Of course, if you're looking to conserve energy and maximize speed on a grand randonee, you might want to go with a tire like the Gatorskin or a Gran Bois (I think they make a 27 or 28mm Cypres).