I have a first-generation Dura Ace sidepull brake on my Austro Daimler Vent Noir.

The front wheel has a fender and a Son Edelux II, both of which mount onto the brake pivot bolt through the fork. The fender and light mounts are thick enough that, with everything on the bike, the brake pivot bolt isn't quite long enough to go completely through the nut on the back of the fork.

I had to remove the concave washer on the back of the fork. But even now I can only get maybe 5 threads of engagement.

So: I wonder if I can replace the pivot bolt in the brake with a longer one.

1. Is that possible?
2. Where does one buy a longer brake pivot bolt?
3. Is there another solution I haven't considered?

I'm not sure about if there are longer bolts out there but there may be. Since you said it was 1st gen Dura Ace I assume you are talking about "nutted" brakes and not a "recessed mount" type of brake.
One work around would be to drill out the back of the fork crown hole to accept the outer diameter of the recessed brake bolt (sorry, I'm at work right now - I don't recall the outer diameter of the recess bolt). Basically you drill the back of the fork crown with a size bit to permit the recessed nut to slide in. Now if your current brake bolt is not threaded that far down (darn-it, I just thought of this)....then you would need to run metric die over the existing brake bolt to thread it down a little more.

This would work but the recessed nuts are available in many different lengths; if one short enough cannot be found the nut can be cut down enough to allow the bolt to thread in sufficiently. IIRC the hole in the back of the fork will need to be enlarged to 8mm. Rule of thumb for minimum thread engagement is equal to the diameter of the bolt, so an M6x1 bolt would need 6 threads engaged; your 5 threads is a little shy of the mark. If you cut a nut short leave at least 6mm of thread, 6 full threads.
Edit: Here is a link for the nuts: https://www.ebay.com/itm/BDW-brake-c...s/232862309847

A little off-topic, but for rear brakes, drilling out only the front part of the rear brake bridge to adapt to recessed nutted brakes is difficult, here is a 3D-printed solution which allows drilling out both sides and adapting back down to the correct diameter: https://www.thingiverse.com/thing:3662440