is this the type of HD enclosure u were talking about?
http://www.endpcnoise.com/cgi-bin/e/00017.html
It even comes with a sticker to tell you how much its overheating.
No, sereously, its a good idea, but might prove to be a little hard to fit in a waterblock.
But a better method might be to go with something like this:
http://www.xoxide.com/2ndhardrivco.html
and just insulate it.
Although, i disagree with their placement. I may be wrong, but wouldnt it be more eficient to mount the block on the flat metal top of the HDD?