My partner and I had a lengthy conversation about Velcro. As a heavy fighter and marshal, he often has a different view on equestrian activities and this leads to good, solid, figure-out-the-logic-behind-the-universe-because-that-seems-mad discussions. I value his experience and knowledge of armour and combat, and it has added a more rounded basis to my fanciful dreamings of gigundo crests the size of wheelbarrows. You have to admit, if someone took the field with a wheelbarrow on their helm, it'd catch your attention. He says it would be a pain.
I am a Special Education teacher by trade and this has given me more than the usual exposure to hook and loop tape. I can tell you how much certain sized rolls are from specific suppliers and which brand does which job best. I have developed an acquaintance with Velcro that my university training never prepared me for. But I have not stuck something to my hat and had someone try to knock it off.
The society rules say 'Velcro or similar methods are recommended' to attach the crest to the helm. The Kingdom rules do not add anything. So, Velcro is recommended and in use by practising riders, but another option would be acceptable as long as it does the job.
I guess most sensible way to proceed would be to talk to people who already use Velcro and magnets, develop a couple of prototypes attachments and see what sort force it takes to knock various crests off using a variety of Velcro sizes.