Tansy is traditionally tucked into your horse's browband if it is bothered by flies.
Tansy would work much like gauze but rather than healing midge bites it would provide you with temporary immunity or resistance before withering. Of course you couldn't wear a hat with nice gilds on it while you had tansy in your hair. This would be directed more to new players for whom midges are a real problem and who would think it worth it.
Alternatively make tansy a gilding object so you could gild a hat with it, and get some permanent lasting protection from midges - no buffs or bonuses, just protection from bugs.