I can’t see the harm in adding a magnet. I’d have done that if I’d thought of it – I’ve got a brilliant magnet I can turn on and off for picking up all the swarf / angle grinding mess from the floor.
Only problem I can foresee with a magnet is getting the filings out.
For what it’s worth, Jim Turnbull recommended against coating the inside of my tank, said it was better to not put anything else in there that could cause issues in future.