I would think that a waterblock would be better since they are purpose built. Your idea sounds pretty cool, but also difficult to pull off (and I bet it would be pretty heavy too).
It's def possible. And it could be a Cu sink. You could build a chamber around the finned portion and have water run over the fins. I just think it would be very hard to get it leak proof, and it would be heavy, because that's a fair amount of water.
Should work fine, but it's likely you can homebrew a better block for about the same price. Or buy a cheap commericial block off eBay. The Maze, TDX and old Swiftech blocks are dirt cheap. There's really no reason to homebrew unless you're really fond of tinkering.
You can make anything work, if you feel the time being put into it will pay off.
For the price of materials, time and labour, coupled with the chances of something going wrong, you'd be better off grabbing a TDX off of the FS/FT forum for ~40$.