Actually, I was thinking of using the fridge brick intact, as a brick with a magnet in it. But you say “pseudo” – does that mean it’s really a non-System object that won’t connect to anything? I’ve never looked at one closely.
Not too long ago LEGO made real 2×4 bricks with magnets inside the three tubes for about $1 each. The more recent fridge magnets, that were made to be minifig stands, do not have tubes underneath its a flat bottom with screws in it so you cannot use any convention LEGO connection on the underside.