Now, I'm no lawyer (I always love saying that!), but if there were a legitimate constitutional defense against Federal marijuana laws, you can be sure a lawyer would have thought of it before now. My guess is that the analogy you make cannot hold up legally. It may have required an ammendment to ban importation, transportation, and sale of alcohol, but not other substances that, unlike alcohol, are legally defined as drugs. Or maybe the ammendment was not actually required in order to enforce a nationwide ban on alcohol, and the temperance movement just decided to go that route? That's probalby not the case --- I have no idea.

My guess is that if you get busted, you should not try this as your defense --- at least don't act as your own attorney!