I thought of both possibilities, Chronos, but didn’t know what to make of them.
Cecil himself said the risk of embolism with non-pregnant women was less well-known, but still advised against taking the risk.
I don’t have any idea how CO2 interacts with blood vessels. My impression is that CO2 injected straight into a vein would be a really bad thing, but I don’t know whether it would be absorbed through the “vein sinuses of the intrauterine wall.” But if an exhaled breath could do the trick, I don’t see why Coke fizz wouldn’t.
Maybe a simpler way of approaching the question would be to ask whether anybody ever really did the fizzy douche thing that snopes describes. It does seem stupid and unlikely to me, which is why I doubted snopes on this count.