Closing this thread out, I can report success on the Gold Rush Randonnee on the IRO. That's 750 miles in 78:46 on one cog (I stayed with the 49x18 for the whole ride), including about a dozen big alpine climbs. I had to walk two short pitches of the Janesville Grade but otherwise made it up (and down!) everything on the ride on the bike.

There were two other guys riding the cog, one from the U.K. and another from Australia and both of them also finished. I rode nearly the whole ride with the fellow from the UK (who was running 48x18) and we rode into the finish together, becoming the first-ever riders to complete that event on a fixed gear.

So, folks, get out there and push youself to do some new, crazy stuff on your fixies. If a guy who first rode a fixie on May 1, 2009 can turn in this kind of ride, then just about anything is possible. Get out and ride!