They have these automated things that are supposed to be for air fresheners for the bathroom that you might be able to use. Glade makes one that is motion activated, but for your purposes one like this would work better. I just found this: http://www.touchfreeconcepts.com/products/time_mist.php
The only problem is that it takes an aerosol can so you'd have to find a way to get your herbal room spray into an aerosol can.