Great question! Unfortunately, I don’t have any specific information on magnets but can say that we have found that strong magnets may cause problems with the air assist fan (the fan that sits on the back of the carriage plate that the printer head moves on.) When the printer head passes over a magnet that is too strong, the fan may slow or stop which would cancel the print.
Another way to keep the material flat on the crumb tray is to use hold-down pins. For example, a lot of customers print these pins available in the free laser design section: Honeycomb bed holdown pins.