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.
Some years ago there were, if I recall correctly, three Spanish flyers who flew under the Ballachulish bridge, a much smaller structure and therefore more challenging and they all came out with a plea of being caught in a downdraft despite them taking it one at a time, so not really very convincing.
But if this Kessock bridge under-flight really occurred someone will have it on video, for everyone has a camera in their pocket these day and that's one reason for being circumspect with such stunts. Good fun I'm sure, but potentially costly in fines and maybe loss of licence.
Wish I'd seen it.