I would assume this question shouldnt be that hard to answer for Apple support. I assume Iphone 12's attract to the magsafe plate 1 of 2 ways.... Either the charging PLATE is a magnet, and the PHONE has ferrous metal in it required for attraction , or, the PHONE has a magnet in it and the PLATE is some type of ferrous metal. I highly doubt the actual phone has the magnet built into it for a number of reasons. (one reason being it would always be picking up little bits of ferrous metal debris that it happened to come across. Therefor the plate is most likely the magnet and the phone has the metal in it. If so, then ANY magnet would attract to the back of the phone. Can anyone confirm this?
If the above is correct, then it will probably come down to the shape and orientation of the metal in the phone. If it is only a ring of metal that aligns perfectly with the outside diameter of the PLATE, (leaving most of the area in the middle for the wireless charging electronics.... then the phone will probably not work with any shape of generic magnet. Or at least it wont be strong enough to support the phone because there will only be small portions of metal ring to plate connecting to eachother. IF the magnet diameter was larger enough to cover the entire ring of metal in the phone then theres no reason why that would not work.