I took a chance and purchased a length of St. Armand Canal White (direct from the mill owner when she was in town). It turns out that this paper is wonderful for palladium and cyanotype, and I suspect for most of the alt processes. It's unbuffered, lightly sized, has good wet strength. It's a little bit more absorbent than Platine or Artistico. Requires no fudging to get the pH balance right. It's a relief to find a paper that just works.
Speaking of Platine, her opinion of the Arches mill is that the art papers division is a tiny piece of an otherwise huge operation. She implied that the supply of Platine is invisible to their management. (She said that one of the top executives she met of Arjomari-Prioux didn't even know they made artists papers).