It would be nice to see this. It could be fairly simple to impliment, just disallow one layer of fluid from a tile per stone in that tile, and put something in the status that reflected this, like 3/7 water, displaced by 4 stones.
Some of you are saying, "so what?"
This would have a number of uses. One, it would make the game more realistic. You have to clear out that tunnel if you want water to flow faster.
Two, it would make the damming of rivers much simpler(especially magma channels), as well as the creation of mid-lake/sea fortresses, while not being a drain on FPS.
Three, you could use them to control reservior overflow. Channel that pond, put a few stones on one side leading to whereever you want the excess water to dump, and forget about flooding forever. Similarly, this could be used to fill ponds to full in succession. Also, you could perhaps use them to create a little rock waterfall set for your dwaves to admire. I know that those ones you see in hardware stores are soothing to me.
Four, you could set up some sort of trap where stones fall in a large body of water, drowning any poor saps nearby.
Of course, there would be drawbacks. It could be very hard to remove these once put in, and you'd have to forbid the stones to keep your dwarves from "borrowing" parts of your dams for their little projects.