posi, interesting that you have a Kuwahara, rare bikes. My old touring bike is a 90 or 91 Kuwahara Caravan, quad butted. My experience with that bike (which I did all my loaded touring on) is similar to what you describe. I'm not a big guy, nor particularly strong, so I dont recall ever seeing the crank move too much, but compared to my aluminum bikes, it is clearly more flexible and one can definitly feel it when pedalling hard. What is clear is that I am nearly always seated, and while one can clearly see and feel how more flexible it is, it still has a nice nice ride. My modern alum cross bike is somewhat more harsh, but not overly so, and is more flexible than my older alum mtn bike that I use with slicks (curved rear stays versus straight ones on the Rockhopper)

I would say that what you are describing Ul, and positron, is probably fairly common. Its hard to say without seeing it how it compares to our bikes, and I understand the concern given that you made the frame and this trip was the first time with some weight on it. I dunno, the fact that you can ride no hands with 35 lbs on it, and it doesnt exhibit any funny business with handling or at speed, would make me think that all is fine.
I guess you could try to compare it to some other steel bikes of similar style, you know, by holding bars and seat with hands, put foot on bb and push abit to see the flex? Kind of like comparing downhill skiis. Would this show a big difference from frame to frame? I don't recall where you did this build, but if it was nearby, I guess the best person to ask would be the frame fellow who did the course for you.
If you werent umpteen 1000 kms from here, Id show you my bike...

All in all, I think Positron summed up things pretty accurately. very good way of putting it and very much close to my experience also.

I shall go and read of your first trip to see how it went.

oh, how was the saddle?

cheers
ps, shorts and t-shirts?? man you guys make me sick (I was on the trainer watching Kung Fu Hustle tonight and it is only -12C tonight)