Wow, some posters can be real hum-bugs, huh?
I think it's a great idea, but you'll have to use way more than a can of Coke to taste anything. I had this great idea too, when I first started brewing. In experimenting to see how much of the flavor would come through, I mixed Coke with Guiness and Anchor Porter in varying quantities. In both cases I had to have at least 1 part coke to 2 parts beer for the Coke to make an appearance in the flavor profile. Roasty flavor is just too much.
When I made my Coke brew, I used a brown porter recipe and still scaled the dark grains back a little. I split the batch after fermentation, just in case the Coke totally ruined it. Then at bottling, I added Coke syrup as the priming sugar. You can get Coke syrup from any eatery with a fountain dispenser in exchange for some of the final product.
I primed 2.5 gal with 1 cup of Coke syrup and it was way to much. No bottle bombs, but the caps were popping like champagne and the beer poured out as solid foam. The other half, primed with cornsugar, was perfect.
The flavor difference was noticable, but not spectacular. Honestly, the plain half is probably the best brew I've ever made. If I did it with Coke again, I would probably add 3 cups of syrup to the secondary for a 5 gal batch.
My recipe provided upon request.