This is in my neck of the woods and it's a tempting thing to do and would, in my opinion, not really be dangerous but I'm sure it would fall foul of the 500ft clear rule.
If you are in this neck of the woods, you will no doubt be well aware that there is often maintenance work being undertaken to the underside of the bridge deck from a moving cradle. It's not uncommon to see steel hawsers, ropes, etc. hanging from it.
And there's plenty of boats passing under with rather tall masts that may be obscured by the bridge pillars until it's too late.