Years ago in Denmark a friend used to bake bread in the French style but containing lots of mixed seeds. Something like a baguette but short and fat with a crusty top. Is there any reason why I shouldn't simply add seeds to my usual baguette dough, change the shape but otherwise follow the usual baguette recipe?
I would not use a generous addition of seeds in an otherwise unchanged recipe because it would be throwing the hydration off. The seeds will absorb some of the liquid in the dough, which then is missing in the dough. It’s hard to give a good ratio of how much extra water the seeds would absorb, this will depend on the kind of seeds, but also on the rising time - in 48h they would absorb more than in just an hour or two. So some experimenting would be advisable.
Especially heavy or pointy seeds will influence the structure of the dough a bit, literally bursting bubbles, but again it’s hard to give a generic answer on how noticeable this will be.
I suggest you start with a small amount of seeds and gradually increase over various batches until you find your preferred balance.
There's no reason you can't add seeds to a baguette dough, it will work fine, the only small consideration is that the seeds will decrease the structure of the dough a little bit.
Stylistically, seeds are associated with heartier breads like whole wheat and rye so seeds may seem out of place on a plain white loaf, there's no technical reason you can't put them in though.