I think the classic ranonneuring bag is a very good way to go, especially if you get a custom bike. The ones that mount to your handlebars are ok, but I've been on a ride where we had to wait for a guy that was trying to fix his handlebar bag. A randonneuring bag sits lower on the bike and the rack and decaleur keep it from moving.

As far as keeping things dry, the classic bags are made of waxed cotton, which is mostly water resistant. I think what you'll find is that most randonneurs use ziploc bags for waterproofing. If you want your clothes to remain dry, you can put them in larger ziplocs or get dry bags. If you are on a rainy 1200k, everything is going to get wet, just the way things work.