This is a subject in which I have been interested for many years. Always having been older and less talented than my riding companions, I've had both to train and ride more efficiently to keep up with them. Pedaling and aero efficiency are the obvious low-hanging fruit as far as mechanics are concerned. Fortunately or unfortunately, depending on how one looks at it, pedaling dynamics are much more complicated than these graphs would suggest. One's subjective feelings of pedaling may not comport well with these graphs. This article explains why:
http://www.me.utexas.edu/~neptune/Papers/essr30(4).pdf
Thus one could knock a cyclist out, clip him in, rotate his pedals at 90 rpm and get pedal force graphs, the shape of which, except for the backstroke, would be difficult to distinguish from the usual graphs which one sees.

So how do we become more efficient? As you've already noticed, the key to seated climbing on a tandem lies in keeping the power output steady during the pedal stroke. This is the reason that many teams can climb better seated OOP than IP. However, as has been noted on the tandem forum, if one has a partner who is sufficiently well trained on a single, one cannot feel any difference with them pedaling OOP or IP. My method has been to train on my rollers to be able to produce a constant hummmm, without bouncing, at any pedal rpm between 50 and 150. Very simple and very effective. One-legged pedaling on my rollers has also proven effective in increasing my measured climbing rate. YMMV with your stoker!