Seems like the tough part of this is access to the magnetizing machine. I wonder if the same effect could be achieved by placing tiny off the shelf magnets[0] in the molded part instead?
While this is possible, it would create stress concentrations within the elastomer and could significantly affect its durability. We saw this effect even when using larger magnetic particles as with ReSkin, https://reskin.dev. If instead we make the elastomer more rigid, it would worsen grasp stability.
I think you could 3d print TPU half shells w/ some reasonable infill (i'm sure there's one with good force transmission characteristics for this?), and then seal the magnet array inside of the two halves.