If I were building a bike up from scratch, using Dura-Ace 10-speed, could I use FSA Ultralight cranks, and if so, which bottom brackets could I use to preserve 10-sp compatibility?
Yes, you can use the cranks, and the BB has nothing to do with drivetrain compatibility. Your frame will determine the correct dimensions of your BB, and your crank will determine the format/standard (isis or Octalink.)
99% of the time, you will need a 68x108/109 bb for this setup on a tri bike.
but
sometimes you will need a 70x108/109 (Italian frame)
or…
68or70x113 if you have a badly designed custom frame…
I’m just confused by the FSA cranks - to use them with DA 10-sp, I don’t think I can use a Shimano BB, because they won’t provide the correct chainline, so I assume I can use a FSA BB, but its not entirely clear.
FSA makes their cranks in ISIS and Octalink formats. The Octalink crank will retrofit onto a current DA/Ultegra/105 9spd BB. Chainline will be close enough to stock to work on a well designed bike.
The Isis crank requires an ISIS BB, which is different from the Octalink. Chainline will be nearly identical with the ISIS crank setup.