There's almost nothing as useless as someone who knows nothing about a topic barging in, but Fred's comment that Arches watercolor paper works well for him emboldens me to ask whether anyone has tried the Arches bright white watercolor paper, my favorite paper for gum. I have no idea in the world how well the bright white would work for platinum, but if the traditional Arches works well for the purpose, in my experience the basic performance of the two papers (again, with gum not platinum) is similar. This paper (hot press) has a very smooth surface that keeps its smoothness through multiple soaks, has a thin crisp quality (compared to other watercolor papers) that makes for fast drying and prints faster than the Fabriano Artistico Extra White in my experience (for gum again). Just on a guess, it seems like it might be worth trying for platinum. If you're not finding a paper that works for you and are testing different papers, you might pick up a sheet of this and see how it works. Nothing to lose but $3 or so.
Afterword: I thought this paper, like the other Arches watercolor paper, came in 90# weight, but at least in the Daniel Smith catalog, the only weights are 300# and 140#. Don't know if that makes a difference.
Katharine