hmmm...a good idea, it really sound similar to this...
http://www.overclockers.com/tips1085/
I am kind suprised on the performace that he got by using this simple setup...by no means is it designed for high overclocking, but if you are looking for quiet, what you are planning sounds like a good option. Funny, I remember talking to a guy about something like this, but he was planning on attaching copper pipes to the side of a all alum case...therefore basically turning the whole case into a radiator. I do think that alum tubing could be used...with some caution. spend the extra $$$ & get them anodized...it will look better & help some fo the corrision concerns. Now if you decided just to use the tubing just for a resevior & have a radiator/fan in there too...sounds like a big res to me & defintaly could not hurt. You would be able to engineer some trick filler cap in the whole thing too. Fun project.