We have not seen St. John's wort used in cocktails, but the authors of Botany at the Bar specifically called it out as a botanical they disrecommend in bitters. 

Additionally, drugs.com [link] has a section on interactions with this botanical:

While you are taking St. John's wort, you must not eat or drink certain foods and beverages that are high in tyramine... Foods that are high in tyramine include: air dried meats, aged or fermented meats, sausage or salami, pickled herring, and any spoiled or improperly stored beef, poultry, fish, or liver, red wine, beer from a tap, beer that has not been pasteurized, aged cheeses, including blue, brick, brie, cheddar, parmesan, romano, and swiss, sauerkraut, over the counter supplements or cough and cold medicines that contain tyramine, soy beans, b sauce, tofu, miso soup, bean curd, fava beans, or yeast extracts (such as Marmite). Caffeine intake should be limited as well. You should avoid or limit the use of alcohol while being treated with St. John's wort. Alcohol can increase the nervous system side effects of St. John's wort such as dizziness, drowsiness, and difficulty concentrating. Some people may also experience impairment in thinking and judgment. Talk to your doctor or pharmacist if you have any questions or concerns.

For these reasons, it would be best to avoid using this botanical in cocktails.