If your res isn't higher than the rest of the system, the high point will collect microbubbles over time. Make sure you can do something with those bubbles.
It's been done before. But that doesn't mean it isn't a good idea! LHGit is right though. Insulate your tubing. It'll bleed off all that sub-ambient temp before getting to your computer and you'll be left with a really complicated method to get ambient temp water to your CPU. You can get insulation for tubing at home improvement stores for plumbing or HVAC.
All in all, your implementation plan sounds quite practical. You might want to place your pump right next to the rad by the AC though. Get as much heat out of the water before you send it back to the computer.
Most importantly, remember to have fun and take pics! After all, if you just wanted lower temps, you could buy a Kryotech system.
