Being that they're no-absorbant, thin metal (which conducts
reasonably well), and have a hole in the middle, it would
seem they are ill-suited towards being coffee mug mats.
Perhaps wind chimes or ???
Have you actually taken the platters out and looked at them?
Not exactly suitable for a sandwich/snack tray.
Since they are not meant for contact with food, the easy
answer is just don't do it. They're "probably" safe if you
washed them off good but I'd not bet on it.
They are almost always nickel-plated neodymium, their
greatest hazzard is violently attaching to something and
scratching it, or a strong magnetic field wiping out a
floppy disc, possibly damaging a watch mechanism... I've
never tried to trash a watch with one though. Their
toxicity should be low but if these things are really a
concern to you, perhaps keeping food on real plates wouldn't
be such a bad idea.