An AIO would work pretty damn well on Titan, but I would be concerned about the VRMs. However, Dwood makes custom brackets where you can mount a fan that will blow directly down onto the VRM area. I did it for my GTX 680 and it came out look pretty good. There were no custom water blocks available for that particular 680 model.
It was well worth it, and a lot of fun! But do note that an AIO will not be within 5 degrees of a real water block (not even close to be honest). But it will perform much better than the stock air cooler. I'd expect it to land somewhere right in between the stock cooler and dedicated water (like ~60C load temperatures).