In my own experience I have found that proper branded rad flush is effective for any hard blockages and light corrosion.
I then smash up half a dozen dishwasher tablets and pour them in. I normally run the car for a couple days and drain and replace coolant.
Things that coke would damage would be mostly the bearing in your water pump on the impeller shaft and also your expansion tank cap when it fizzes. Of course having never actually tried this I can't say for sure but honestly you would be cheaper and I imagine more effective spending a fiver on a flushing agent and nicking half a dozen tablets from the Mrs/somebody else's mrs.
I agree with your logic though that coke should be good. If you want to find out without risk then try leaving an old water pump in coke to see? Or fill an old rad up out of the car I guess.
