No experience but would have thought calmers would be worth a go. Narcolepsy in humans is triggered by emotions so thinking keeping the horse very level headed would help.
As for shivers I think some feed studies were done in America.
Narcoleptic horses usually collapse when they are already relaxed, or anticipating something such as food, or being girthed up. There are drugs to help this, although I doubt many are actually treated, but as far as I am aware, nothing in the herbal end of the market. You could try a homeopathic vet maybe?