I'm looking for some expertise. I'm mounting a Dura Ace 7800 brake on a
Pinarello fork (my first winter project bike) and the brake's pivot bold is
about 10 to 15mm too short to reach the sleeve nut when inserted into the
rather deep carbon fork. The same is true for the rear brake.

What's the standard solution to this? Does Shimano make accessory pivot
bolts that are longer for this situation??

No, all you need a longer recessed nut made especially for carbon fiber forks.
The standard nut, as you have discovered, is 10mm long. The longer ones are
available in 20mm and 22mm. If your LBS doesn't have them, you can get them
from Loose Screws. Search their website for "brake/recessed nuts":

http://www.loosescrews.com