I'm in agreement with the general response of "you can't shift out of the saddle with DT shifters".
I'm also here to comment that with the freewheels and chains of the 70's, you'd have a hard time shifting under load, regardless of whether you were standing or sitting. It wasn't until Hyperglide sprockets came along that you had much chance of forcing a shift under load, and that was mostly a result of the mountain bike portion of the business (as best as I can recall, of course).
Perhaps it is possible to be pedaling while standing and still take pressure off of the pedals to shift?? I certainly haven't tried.
Planning ahead is certainly the best strategy for hill climbing. I can recall times on the local 14% grades where I was pedaling slowly and trying to get onto a smaller chainring (without shaped teeth or ramps or pins, etc). It became clear that the front derailleur spring had no chance of moving the chain off of the current ring with a lot of tension on the chain. Modern rings and tooth profiles might have a chance??
Steve in Peoria (we don't have steep hills... we have steep river valleys)