Amazing! I would have never guessed there was so much variation in battery technologies available for sale.
I guess finding a reliable manufacturer will be a problem, they're such a 'bargain basement' technology that I suspect even a lot of the branded varieties of ZnCl batteries are probably just resleeved cells, bought at the cheapest price possible that month.
Having stolen a magnet off the fridge & raided the back of the cupboard for cells, it appears that the Tesco Value AA batteries have a ferromagnetic top + bottom contact, whereas the AAA cell (or at least, the one that I have!) has an only slightly magnetic bottom contact. I can get my walking compass to deflect perhaps tens degrees with it, compared to going entirely haywire as with a steel cased Alkaline.
Sounds like you might be on to a winner with the Populars then!
I'm surprised no one has tried this before, but I guess that's always the way!