Why This Works
The black and tan is a demonstration of density: the nitro-style stout is lighter than the paler beer below it, so poured gently it floats in a clean line. Alcohol never had anything to do with it.
The technique is everything. Pour slowly over the back of a spoon and let the stout settle; rush it and the layers muddy into one.
Use a stout with a creamy, nitro-style head if you can. The texture is half the pleasure, and non-alcoholic stouts have gotten genuinely good at it.
The Alcohol-Free Swap
This one needs two beers: a clean non-alcoholic pale ale or lager for the base and a rich non-alcoholic stout to float. The bigger the contrast, the better the effect.