Free swinging weights? Pendulum. Like on a Grandfather clock? Is that the motion your describing, to drive up and down the piston? A large sandbag swinging back and forth, with a side arm, pulling and pushing up and pulling down, a bobbing piston? Displacing the motion of a free swinging sandbag. Is that what ya meant? In water this motion causes ripples. Those ripples could and should provide an area of effect nova-like wave on the liquid, creating a circular wave. A great way to trigger more than one pressure plate at a the exact time or in precise intervals.
I could get really cheerful about this suggestion. Especially if it were a Foucault's pendulum using the earth's power to record or move minuscule sequential block, or keep ticking recordings. Something like this would be a unending motion. It sure would be nicer to just make permanent motion a feature of the fantasy world in this fashion, as opposed to some space-whoring, mechanical nightmare of windmill-pump design.
A good suggestion. *thumbs up*
As for Rigs, Pulleys and Weights. I would add Chain Wench. And that's not to be confused with a female slave on a noose. And call it all Tackle, except for the weight. Game already has a very nice weights system, blocks, it just needs code in place to make coffers, mobile.
Sincerely,
Knutor