Ah, an aqueous soldering approach. It's just an application of "electroplating" - remember that electricity isn't strictly required to make the reaction work, it just shifts the equilibrium point and makes it much faster.
It's a good idea. The obvious downside is you'd have a hell of a time ensuring even evaporation. Any trapped pockets of water will drastically reduce efficiency of the cooling interface, along with other problems. You'd also have to constantly add more (aqueous dissolved) copper into the solution at a controlled rate, as the molar concentration will keep on diminishing while it deposits. Any impurities in the water would likely turn into Slightly Bad Things. I imagine you'd want to apply heat (perhaps 100C water boiling heat) to finish the join before x-mas.
I think it's possible. It might be easier (more economical) to simply copper braze the two parts together, or machine them from a single block in the first place, then attach to processor afterwards. Probably won't get as good as just tossing the IHS out the window.