Bonnets are compound curves so, unless very small, there is no way that a mag is going to sit flat. If it isn’t flat then the wind is going to get under it.
As above, I’d advise against, then it becomes his choice, but I’d word my invoice carefully. Something like "magnetics for use on sides and rear of van only" then if he does do it, and come unstuck, you should be immune to any claim of him denying that he was warned.
It’s a good question, because we make magnetics for people and never ask what they are going to be used for. Perhaps we all need to be more careful.