I've used molasses in the past to carbonate in recipes where I already have molasses added for flavor. There is just not enough added for carbonation to really give any residual flavor, so I usually stick to table sugar for carbonation. It is cheap, easy to get, and ferments out completely.
If you do try molasses, you can stick it in the microwave for a few seconds to warm it slightly to reduce the viscosity and allow it to pour a bit easier.