"The answer might just be that there is no perfect bike for balancing those two things... ce'st la vie."

Bikes get sold from within categories, so yeah, you have to choose. The LHT is perfect for a Trans Am, and while that ride is going on, you will be so much more deeply into that experience, that I think you will thank yourself for having given that experience the respect it deserves. You will be so much more immersed in a trans am compared to a lot of subsequent short rides around town, or whatever, that I think that should be your focus. Plus, if after the ride is over you decide you need a time trials bike, you should be able to get out of the Surly to move onto whatever you want, good resale.

To me the bigger issue is what load level you guys want. The whole ultralite backpacking deal, where the average guy is getting his core weight down to about what my rear panniers weight empty, is some thing anyone contemplating a tour like yours should consider. Partly because to some extent the average bike tour gear is apparently ignorant of the trend and remains heavy load oriented, and partly because the disparity in ages of your group could mean you have at least one rider who really needs to be load conscious.

When I was in the biz back in the early 80s, we carried special ultralite gear at the backpacking/canoeing store specifically for cyclists, who were deemed to have special lightweight needs. That seems like an odd assumption considering how much easier it is to carry a load on a bike, but it was the way it was then. Today the bike ethos is sorta heavy gear oriented. The idea of a touring bike rests on a number of pillars, but certainly one of them is the need to carry a lot of gear. If one dumps that, then the average bike requirement shifts, and a touring bike is even harder to market than it is now.

Certainly there are reasons to carry a little more gear on a bike. Bikes lack shade, and one uses a lot of water, so carrying water is a factor. Also the minimal tool kit (split 3 ways in your case) can add up on longer trips. On the other hand there is generally less need for food, as it can often be resupplied hourly.

That said, while keeping the load down, I would still go for a touring weight frame, for the geometry, and the BOs. The other gear like brakes and gears is often pretty much standard, and I ride light but strong wheels regardless. Just don't think the frame weight will make that much difference. I'm assuming you are normal stature. The average guy gets such an efficiency boast compared to what a woman gets with a bike nearly the same weight, that I don't really think it is that big a deal.

There are some tips here, even if he isn't quite a bike snobby enough for most of us:

http://rayjardine.com/adventures/2010-TransAm/index.htm

One last point is that to the extent you can all get together on this, the better. The more similar the basic platform, the more you will ride the same speed, and split as many common items as possible. Less optimal would be say, one rider on ultralite gear, another set up for expeditions, and a third insisting that bike he delivered papers on in the 70s was all he needed...