Can a Navigator place a trade current within S of a friendly ship that is lost in a fog bank? If so, is it measured from any part of the fog bank?
I think you’re correct that they can’t. The way I understand it, lost ships are treated as if they don’t have a definite location on the table; placing them inside the fogbank is merely a convenient way to mark which fogbank they are lost in.
This is correct. “Lost” ships have no defined position, so you can’t use them as a basis for anything that’s range-based, even if the thing could completely envelop the fog bank.
The things that do still work while a ship is “lost” are those that can target globally, like the Duel or Mermaids events, Copiers, Mycron, or OE Davy Jones when he rolls a 5 or 6.