I found the image here and thought it suited this well:
http://johneaves.wordpress.com/2009/04/23/conan-and-cobb/As with all things its going to turn out that someone's suggested this already, I did find a passing mention under "better mechanics" but that sounded more like a pump which produces power when a dwarf is working it which while similar doesn't give the same satisfaction as enslaving the goblins which attack your fort.
So anyway, down to what I was thinking of:
I was thinking it would be kinda cool if there was a building much like a windmill but which needed one or more creatures to power it.
How much power it generates depends on how powerful the creatures pushing it are.
You could treat the building much like a chain now except that those attached to it are compelled to walk around and around.
Perhaps link how quickly creatures become exhausted to how much of the power is being used so that a single goblin powering a few pumps would become exhausted and collapse quickly while a titan wouldn't even notice the same load.
It would probably make it more complex but perhaps a dwarf with the Slave Driver job (using the whip as it's tool) might have to occasionally drop by and motivate the work force or else the power being produced drops off.
I've been trying to keep this as close to workshops etc which already exist since I know it's easier to copy/paste old code than try to get something working from scratch.
Sound ok?