I've done 200-600km brevets on various bikes, Rivendell and others. My first 200km was on a Heron road bike, and I just last week set my personal best time for 200km on that same bike. On the first one I had a saddle bag, and that was the last time I did a brevet without a handlebar bag! The most similar Riv models to the Heron road would be Roadeo, Rambouillet, or Roadini. I have done a large amount of brevet riding on a Bleriot, and the "country bike" line descended from that (A Homer Hilsen, Sam Hillborne) would be similar.
As others have said, comfort is the priority, but I would be more specific and say that comfort _over many hours_ is the priority. A lot of things, like cushy saddles and cushy shoe soles, are comfortable feeling at the start, but after 8 hours of riding can be the source of much agony. I would put your riding position at the top of the list, and everyone is different in that regard. I am light and flexible and the most comfortable position for me for very long rides is the classic road bike position, back at about 45 degrees, drop bars with the tops almost even with the saddle and a pretty deep drop so my position changes a fair amount as I move around on the bars. One of the important lessons I have learned is that riding too slowly can be a problem, just as trying to ride too fast can be a problem. They are different kinds of problems, to be sure. For me, riding too slowly means 1) I'm in the saddle longer (duh!), and 2) I'm putting more weight on my butt and less on my legs. I have many times been struggling in the last half of a brevet and found that spending some time riding a bit faster has helped me to feel better. Having the ability to change between a broad range of positions is important for me, and all my bikes, despite their differences in tubing and geometry, wind up with very similar rider positioning.
Back to the handlebar bag ... for me that is a very important part of the setup. While I love the way Grant's designs ride and handle, they tend to have fairly high steering flop that is amplified by the weight of a handlebar bag. I don't usually carry a lot of weight there, and I mostly ride relatively gentle hills,so it's not a deal-breaker. If I was doing a lot of curvy mountain descents, I wouldn't choose a Rivendell with a handlebar bag that had much weight in it. That's one of the reasons I ended up replacing the Bleriot with a bike with less trail. I took the Bleriot (which was my travel bike) down some descents that would have been more fun with more linear steering response. Also, I had my Heron fork pulled for more rake, less trail/flop.
Lastly - tire sizes (carrying over from another recent thread!) - with the wide availability of truly excellent tires in 622 and 584 diameters and a wide range of sizes, it's almost irrelevant what size wheels the bike uses. If your brevets involve rough roads/gravel/dirt, of course you will want pretty wide tires. You probably want decent fenders, too, if there's any chance of it raining. I've done brevets on tires ranging from 26mm to 54mm wide. Wider is generally better, but I've had good and bad rides on all of them. All Rivendells tend to have more tire and fender room than other bikes designed for similar purposes.
Ted Durant
Milwaukee, WI USA