Just from personal experience, I remember my Mom having a lot of trouble getting 'carries' here in Canada. She had to travel a fair distance to a specific clinic daily for a number of months before her dosage was sufficiently low enough to be allowed a week's worth or so.
Unfortunately, I don't know anything about Methadone, or horses, or how the two might interact, so I can't tell you about your specific question.
How much methadone does it take to kill a horse? And what country are you in?