The Oatstraw infusion sounds soooo soothing.
Are you thinking strictly herbs? My thought was to have on hand either powdered milk or buttermilk on hand. It's soft and soothing and wonderful with a little honey too.
Hmmmmm, how about something that infuses more readily like Chamomile or Lavender blossoms? I have used Yarrow in the bath before, mostly for people who are itchy from excema or ivy, but it could be useful for acne too, or just plain nice to bath in
I personally don't like soaking in herb vinegar because I feel like a bowl of salad.
Also, maybe you could make some oatstraw infusion and freeze it in paper cups that way you can just plop one in when you're ready to bathe? just a thought.