Does there formally exist such a thing as a "mountain flying ticket"? If so, by whom is it regulated, and issued? As an endorsement to what license?
Not yet, but EASA are working on it. It is meant to be based on the existing French and Swiss mountain ratings, which focus on landing at high-altitude one-way strips and on glaciers. I believe EASA will bring in currency and re-validation requirements, which will be an administrative bore, but on the other hand they will let you train for the rating anywhere in Europe (Holland? Denmark?) which will make it more accessible.