So I decided to try to find out.
Here's the equipment. Cheap cider, two empty plastic containers and 0.5mm copper wire.
And here they are in situ.
The idea being, slugs and snails would be attracted to the cider. If the copper barrier theory has any validity, the right hand pot would fill with dead specimens, the right hand with its copper wire, though attractive, would be snail free, or at least have less than the right hand one.
Unfortunately, some other allotment fauna, likely a fox, likes cider so much it pulled up the pots apparently to get the last dregs of it out of them.