Magnetic drain plugs are near worthless IMHO. They're not going to attract any crankshaft bearing or cam bushing overlayment as that is non-ferrous material.
If you have enough ferrous particles stuck to a magnetic engine drain plug to worry about then metallic attraction is the least of your worries as this means the rings/cylinder walls are gone and you should have been burning oil.
If a transmission, automatic or manual, has enough ferrous particles to worry about then the transmission is on the way out anyway. Don't go far from home.