Last time I went mtn biking, i got a bad dose. I think i smacked against a large branch that was hanging across the trail. The thing I found over-the-counter that works for me is Burt's Bees Poison Ivy Soap. Active ingredient is Jewelweed, with a base of clay and pine tar. Whenever the itch gets bothersome, I lather up the area and let the soap dry. No itching for 4-8 hrs. It's not a miracle drug and I don't think it heals the posion ivy any faster, but for immediate relief from itching, it's the thing for me.
Sorry, OP, I know this doesn't address your original question, but if anybody out there's suffering from PI and this helps them out...