I've never injected butts with coke, but I have used dr pepper, cherry dr pepper and barqs rootbeer. I do let it go flat and then add a tablespoon or two of rub.
The rootbeer had the most noticable taste but it was not overwhelming. Since butts are already fatty, there really is no need to inject unless you are trying to get the flavor deep into the meat.
I say give it a try. I will not hurt the end result. For me, it's always fun to experiment w new techniques. Some work and some don't.
Bubba Keg / Stumps Baby / Super Fast Yellow Thermapen / Lodge Cast Iron Grill / Smoke Vault