The use of marbles is pretty common here (in Holland), especially for mares that compete in high levels and for moody mares. It works for most of them and I've known mares that have had a marble for years without problems.
I've ridden a mare in dressage that could hardly be handled when in heat, she would throw herself against the wall of her box if you touched her sides, riding when she was in heat wasn't even an option. After placing a marble she was a different horse, and she kept the marble for 4 years, then it was removed because the owners wanted to breed her. They did, and have bred her every year since. As long as she's pregnant, she's calm, but she's still a disaster when in heat.

HUP HOLLAND HUP