If you've got a (mostly) symmetrical action, like bipedal walking, there almost certainly is some way to recapture "returned power" from footfalls to reuse for the next step. It'd be a little different from just balancing reciprocating pumps against each other, since you'd need to "time slip" the recaptured energy - but a flywheel might make that possible without incurring too much loss to make it worthwhile.

For multi-legged robots, a bit of smart geometry can pretty much do this already - see these things: http://www.strandbeest.com/

I suspect using some computer-adjustable versions of the "multipliers" from the article would allow you to build something like a strandbeest which you could steer while only losing a little of the efficiency.