Agree a pulse ox trend to predict AMS in advance is difficult.
But..just to add to the discussion (although this was not the question), there is another use for it, especially at altitudes higher than Whitney, or with a rare Sierra HAPE case:
sometimes people lagging behind with "Mountaineer's Foot" (can't put one in front of the other) actually may be much worse off than we realize. So if their O2 sat is 70 and everyone else is 90, descent and/or evacuation which is already necessary is now seen as immediately imperative. That might (should!) change the plan.
