It's not strictly 'on plan', but I've never found a problem with having a carbonated drink every now and then. Not only Coke Zero, but Pepsi Max, Diet Coke, and all manner of diet sodas that do actually contain citric acid.
I don't advocate that for everyone, though - a few people can be more sensitive to it than others, and the reason citric acid isn't advised is that too much of it stalls ketogenesis in the body by reactivating something called the TCA cycle... I won't bore you with the details! As with all things, moderation is the key.
However, I digress, since you're not asking about citric acid: so yes, one Coke Zero will probably be fine. I take a realistic view of these things, in that if it's going to keep you motivated and happy to carry on, then it's by far the better option than feeling deprived and failing.